Polymerizable compound, polymer, ink composition, printed articles and inkjet recording method

ABSTRACT

A polymerizable compound represented by the following formula (1): 
     
       
         
         
             
             
         
       
         
         
           
             wherein, in the formula (1), R 1  represents any one of a hydrogen atom, a substituted alkyl group, and an unsubstituted alkyl group; R 2  represents an alkylene group; W represents any one of —CO—, —C(═O)O—, —CONH—, —OC(═O)—, and a phenylene group; X represents any one of —CO—, —NHCO—, —OC(═O)—, —CH(OH)CH 2 —, and —SO 2 —; R 3  and R 4  represent a monovalent substituent; R 3  and R 4  may be bound to each other to form a ring structure; and m and n each independently represent 0 or 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymerizable compound and a polymer.In particular, the present invention relates to a novel polymerizablecompound that is very useful industrially, and a polymer that comprisesa copolymer unit derived from the polymerizable compound and is used asa pigment dispersant to be used for color filters, inkjet ink, etc. Thepresent invention also relates to an ink composition suitably used forinkjet recording, a printed article using the ink composition, and aninkjet recording method using the ink composition. In particular, thepresent invention relates to: an ink composition suitable for inkjetrecording, the ink composition being superior in the dispersibility of acolorant so that it has excellent coloring property, being cured byirradiation with an active radiation ray without generating volatilecomponents and enabling the formation of a high quality image; a printedarticle obtained using the ink composition, and an inkjet recordingmethod using the ink composition.

2. Description of the Related Art

As an image recording method for forming an image on a recording mediumsuch as paper based on image data signals, there are anelectrophotographic system, sublimation-type and melt-type thermaltransfer systems, an inkjet system, etc. In particular, the inkjetsystem is applicable to an inexpensive apparatus, and performs directimage formation on a recording medium by ejecting ink only to imageareas where ink deposition is necessary; therefore, the inkjet systemuses ink effectively, thus reducing the running cost. Further, theinkjet system generates less noise and is a superior image recordingsystem.

The inkjet system enables printing on recording media that do not absorbwater, such as plastic sheets and metallic plates, as well as on paper.In the inkjet system, it is an important target to achieve higherprinting speed and higher printing quality, and the time that liquiddroplets take to dry and cure after printing has significant influenceon the sharpness of the image. In one inkjet system, an inkjet recordingink that can be cured by irradiation with an active radiation ray isused. According to this system, sharp images can be formed by curing inkdroplets by irradiation with an active radiation ray immediately afterprinting.

In order to form highly accurate images with excellent coloringproperties, the curable ink composition in such an inkjet recording inkthat can be cured is required to have high dispersibility of a pigment,and stability over time. Reduction in the pigment size is necessary forimparting clear tone and high tinting strength to the ink composition.In particular, ejected droplets of the inkjet ink exert a significantinfluence on the sharpness of images; therefore, the ejection quantityof the ink droplets needs to be small, and the size of the pigmentparticles contained in the ink has to be smaller than the thickness ofthe film formed by curing of the ink. However, when the size of thepigment particles is reduced so as to achieve higher tinting strength,the pigment particles are hard to disperse, and pigment aggregates areeasily formed. Another problem is created in that the viscosity of theink composition is increased by the addition of a dispersant. Theformation of the pigment aggregates and the increase in the viscosity ofthe ink composition both adversely affect ink ejection property, andsuch an ink composition is not preferred.

When an ink composition is used in inkjet recording, the ink compositionis contained in a cartridge. The ink composition in the cartridge isheated at ejection and cooled at a non-ejection time and at storage; inthis way, the ink composition undergoes repeated temperature changes(heating-cooling). This temperature changes also adversely affect thepigment dispersibility, and the pigment dispersibility is deterioratedwith time, causing problems that pigment aggregates are easily formed,or the increase in the viscosity of the ink composition easily occurs.

Accordingly, there is a demand for an ink composition having sufficientfluidity and excellent stability of the pigment dispersion over time, inwhich fine colorant particles are dispersed stably. Various proposalshave been made concerning dispersants for achieving stable pigmentdispersion liquids.

In order to improve compatibility with the pigment, an ink compositionusing a pigment derivative as a dispersant (see, for example, JapanesePatent Application Laid-pen (JP-A) Nos. 2003-119414 and 2004-18656); anink composition that uses a polymer having a basic group as a dispersantfor a specific pigment such as a phthalocyanine-based orquinacridone-based pigment (see, for example, JP-A No. 2003-321628); andan ink composition which is free of organic solvent and contains adispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid)graft polymer and a specific monomer that dissolves the dispersant (see,for example, JP-A No. 2004-131589) have been proposed.

In these ink compositions, the dispersion stability of pigments isactually improved owing to the function of the dispersant. However, thesize of the pigment used in the ink composition is not sufficientlysmall, and there is a room for improvement of dispersibility of finerpigment particles. Further, the ink composition also has a problem inthat the dispersion stability upon long-term storage or upon repeatedtemperature changes is still insufficient.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to solve the conventional problemsand to achieve the following objects. Specifically, an object of thepresent invention is to provide (1) a novel polymerizable compound thatis very useful industrially, (2) a polymer that includes a copolymerunit derived from the polymerizable compound and is used as a pigmentdispersant which improves the dispersibility of a fine pigment andstability thereof, (3) an ink composition suitable for inkjet recording,which can form a high quality image with clear tone and high tintingstrength and can be cured by irradiation with an active radiation raywithout generating volatile components, (4) a printed article obtainedusing the ink composition, and (5) an inkjet recording method using theink composition.

As a result of dedicated investigations conducted by the presentinventors, they have found a polymerizable compound having a specificnitrogen-containing heterocyclic structure, and have found that use ofthe polymer that includes a copolymer unit derived from thepolymerizable compound (monomer) as a pigment dispersant can solve theproblems, and thus the present invention has been made.

Means for solving the above-mentioned problems are as follows.Specifically,

<1> A polymerizable compound represented by the following formula (1):

wherein, in the formula (1), R¹ represents any one of a hydrogen atom, asubstituted alkyl group, and an unsubstituted alkyl group; R² representsan alkylene group; W represents any one of —CO—, —C(═O)O—, —CONH—,—OC(═O)—, and a phenylene group; X represents any one of —CO—, —NHCO—,—OC(═O)—, —CH(OH)CH₂—, and —SO₂—; R³ and R⁴ represent a monovalentsubstituent; R³ and R⁴ may be bound to each other to form a ringstructure; and m and n each independently represent 0 or 1.

<2> The polymerizable compound according to the <1>, which isrepresented by the following formula (2):

wherein, in the formula (2), R³ and R⁴ represent a monovalentsubstituent; and R³ and R⁴ may be bound to each other to form a ringstructure.

<3> A polymer including a copolymer unit derived from a monomerrepresented by formula (1).

<4> The polymer according to the <3>, further including a copolymer unitderived from a monomer represented by formula (2).

<5> The polymer according to one of the <3> and <4>, wherein the polymeris a graft copolymer containing, as a copolymer unit, a polymerizableoligomer (macromonomer) having an ethylenically unsaturated double bondat a terminal thereof.

<6> An ink composition including: the polymer of any one of the <3> to<5>; a polymerizable compound (a); and a pigment (b).

<7> The ink composition according to the <6>, further including apolymerization initiator (c).

<8> The ink composition according to the <7>, wherein the polymerizablecompound (a) is a radical polymerizable compound, and the polymerizationinitiator (c) is a photo-radical generator.

<9> The ink composition according to the <7>, wherein the polymerizablecompound (a) is a cationic polymerizable compound, and thepolymerization initiator (c) is a photo-acid generator.

<10> The ink composition according to any one of the <6> to <9>, whichis used for inkjet recording.

<11> A printed article obtained by curing the ink composition of any oneof the <6> to <10>.

<12> An inkjet recording method including: ejecting the ink compositionof any one of the <6> to <10> on a recording medium using an inkjetprinter; and irradiating the ink composition ejected with an activeradiation ray to cure the ink composition.

The present invention can solve the conventional problems and canachieve the above object. By using a high-molecular compound that givesexcellent dispersibility and dispersion stability, the present inventioncan provide an ink composition suitable for inkjet recording which issuperior in the dispersibility of fine pigments and in the stability ofthe dispersion, enables the formation of a high quality image having asharp color tone and high tinting strength, and can be cured byirradiation with an active radiation ray.

In addition, the present invention can provide a printed article with ahigh quality image which was obtained using the ink composition that canbe cured by irradiation with an active radiation ray and provide a sharpcolor tone and high tinting strength.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

(Polymerizable Compound)

The polymerizable compound of the present invention is represented bythe following formula (1).

<R¹ in Formula (1)>

In the formula (1), R¹ represents a hydrogen atom, or a substituted orunsubstituted alkyl group. The alkyl group has preferably 1 to 12 carbonatoms, more preferably 1 to 8 carbon atoms, still more preferably 1 to 4carbon atoms. Examples of the alkyl group include a methyl group, anethyl group, a propyl group, an n-butyl group, an i-butyl group, at-butyl group, an n-hexyl group, a cyclohexyl group, a 2-hydroxyethylgroup, a 3-hydroxypropyl group, a 2-hydroxypropyl group, and a2-methoxyethyl group.

<R² in Formula (1)>

R² represents an alkylene group and has preferably 1 to 12 carbon atoms,more preferably 1 to 8 carbon atoms, still more preferably 1 to 4 carbonatoms. Examples of the alkylene group include a methylene group, anethylene group, a propylene group, a trimethylene group, and atetramethylene group.

<W in Formula (1)>

W represents —CO—, —C(═O)O—, —CONH—, —OC(═O)—, or a phenylene group.

<X in Formula (1)>

X represents —CO—, —NHCO—, —OC(═O)—, —CH(OH)CH₂—, or —SO₂.

<m and n in Formula (1)>

m and n each independently represent 0 or 1.

<R³ and R⁴ in Formula (1)>

R³ and R⁴ represent a monovalent substituent. R³ and R⁴ may be bound toeach other to form a ring structure.

The monovalent substituent for R³ or R⁴ is preferably an alkyl group, anaryl group, a halogen atom, an alkoxy group, an aryloxy group, analkylthio group, an arylthio group, an alkoxycarbonyl group, anacylamino group, a carbamoyl group, a cyano group, an alkylsulfonylgroup, an arylsulfonyl group, an acyl group, or a sulfamoyl group, andmore preferably, a halogen atom, an alkyl group, an aryl group, analkoxy group, an aryloxy group, an alkylthio group, an arylthio group,an acylamino group, or a sulfonylamino group.

The substituent represented by R³ or R⁴ may be unsubstituted orsubstituted. For the substituent that can be introduced into thesubstituent, an alkyl group, an aryl group, a halogen atom, an alkoxygroup, an aryloxy group, an alkoxycarbonyl group, an acyloxy group, anacylamino group, a carbamoyl group, a cyano group, a carboxyl group, asulfonyl group, and a heterocyclic residue are preferable.

The alkyl group for R³ or R⁴ is preferably an alkyl group having 1 to 30total carbon atoms, more preferably, an alkyl group having 1 to 20 totalcarbon atoms. Specifically, a methyl group, an ethyl group, a butylgroup, a hexyl group, an octyl group, a 2-ethylhexyl group, a3,5,5-trimethylhexyl group, a dodecyl group, an octadecyl group, abenzyl group, a (4-ethoxyphenyl)methyl group, an N,N-diethyl carbamoylmethyl group, an N,N-dibutyl carbamoyl methyl group, a 1-(N,N-dibutylcarbamoyl)ethyl group, a 2-methoxy ethyl group, a1-methyl-2-phenoxyethyl group, a (4-chlorophenyl)methyl group and a(2,4-dichlorophenyl)methyl group are preferable, and an ethyl group, abutyl group, a hexyl group, a benzyl group, an N,N-diethyl carbamoylmethyl group, an N,N-dibutyl carbamoyl methyl group, a 1-(N,N-dibutylcarbamoyl)ethyl group, a 1-methyl-2-phenoxyethyl group, a(4-chlorophenyl)methyl group and a (2,4-dichlorophenyl)methyl group aremore preferable.

The aryl group for R³ or R⁴ is preferably an aryl group having 6 to 30total carbon atoms, more preferably, an aryl group having 1 to 20 totalcarbon atoms. Specifically, a phenyl group, a 2-methylphenyl group, a3-methylphenyl group, a 4-methylphenyl group, a 4-phenylphenoxy group, a4-chlorophenyl group, a 2-methoxyphenyl group, a 3-ethoxyphenyl group, a4-butoxyphenyl group, a 2,4-diethoxyphenyl group, a 2,5-dibutoxyphenylgroup, a 4-phenoxyphenyl group, a naphthyl group, a 4-dibutyl carbamoylphenyl group and a 4-dibutyl sulfamoyl phenyl group are preferable, anda phenyl group, a 2-methylphenyl group, a 3-methylphenyl group, a4-methylphenyl group, a 4-chlorophenyl group, a 2-methoxyphenyl group, a3-ethoxyphenyl group and a 4-butoxyphenyl group are more preferable.

The halogen atom for R³ or R⁴ is preferably a fluorine atom, a chlorineatom, a bromine atom or an iodine atom, and more preferably a fluorineatom or a chlorine atom.

The alkoxy group for R³ or R⁴ is preferably an alkoxy group having 1 to30 total carbon atoms, more preferably, an alkoxy group having 1 to 20total carbon atoms. For the substituent, a methoxy group, an ethoxygroup, a propoxy group, a butoxy group, a hexyloxy group, a2-ethylhexyloxy group, a 3,5,5-trimethyl hexyloxy group, an octyloxygroup, a decyloxy group, a 2-phenoxyethoxy group, a2-(3,5-di-t-butylphenoxy)ethoxy group, a dibutyl carbamoyl methoxygroup, a hexadecyloxy group and an octadecyloxy group are preferable,and a methoxy group, an ethoxy group, a butoxy group, a hexyloxy group,a 2-ethyl hexyloxy group, a 3,5,5-trimethyl hexyloxy group, a2-phenoxyethoxy group and a dibutyl carbamoyl methoxy group are morepreferable.

The aryloxy group for R³ or R⁴ is preferably an aryloxy group having 6to 30 total carbon atoms, more preferably, an aryloxy group having 6 to20 total carbon atoms. For the substituent, a phenoxy group, a tolyloxygroup, a 4-chlorophenyloxy group, a 4-acetamide phenyloxy group, a2-butoxyphenyloxy group, a 2-benzoyl aminophenyloxy group, a2,5-dimethoxy-4-nitrophenyloxy group and a 3-octyloxy phenyloxy groupare preferable, and a phenoxy group, a tolyloxy group, a4-chlorophenyloxy group, a 4-acetamide phenyloxy group, a2-butoxyphenyloxy group and a 2,5-dimethoxy-4-nitrophenyloxy group aremore preferable.

The alkylthio group for R³ or R⁴ is preferably an alkylthio group having1 to 30 total carbon atoms, more preferably, an alkylthio group having 1to 20 total carbon atoms. For the substituent, a methylthio group, anethylthio group, a butylthio group, a hexylthio group, a2-ethylhexylthio group, a 3,5,5-trimethylhexylthio group, an octylthiogroup, a decylthio group, a 2-phenoxyethylthio group, a2-(3,5-di-t-butylphenoxy)ethylthio group, a dibutyl carbamoyl methylthiogroup, a hexadecylthio group and an octadecylthio group are preferable,and a methylthio group, an ethylthio group, a butylthio group, ahexylthio group, a 2-ethylhexylthio group, a 3,5,5-trimethylhexylthiogroup, a 2-phenoxyethylthio group and a dibutyl carbamoyl methylthiogroup are more preferable.

The arylthio group for R³ or R⁴ is preferably an arylthio group having 6to 30 total carbon atoms, more preferably, an arylthio group having 6 to20 total carbon atoms. For the substituent, a phenylthio group, atolylthio group, a 4-chlorophenylthio group, a 4-acetamide phenylthiogroup, a 2-butoxyphenylthio group, a 2-benzoyl aminophenylthio group, a2,5-dimethoxy-4-nitrophenylthio group, a 3-octyloxy phenylthio group arepreferable, and a phenylthio group, a tolylthio group, a4-chlorophenylthio group, a 4-acetamide phenylthio group, a2-butoxyphenylthio group and a 2,5-dimethoxy-4-nitrophenylthio group aremore preferable.

The alkoxycarbonyl group for R³ or R⁴ is preferably an alkoxycarbonylgroup having 2 to 30 total carbon atoms, more preferably, analkoxycarbonyl group having 2 to 20 total carbon atoms. For thesubstituent, a methoxycarbonyl group, an ethoxycarbonyl group, abutoxycarbonyl group, a phenoxycarbonyl group, a (2-ethylhexyl)oxycarbonyl group, a hexyloxycarbonyl group, an octyloxycarbonyl group,a (4-methoxyphenyl) oxycarbonyl group are preferable, and amethoxycarbonyl group, an ethoxycarbonyl group, a butoxycarbonyl groupand a phenoxycarbonyl group are more preferable.

The acylamino group for R³ or R⁴ is preferably an acylamino group having1 to 30 total carbon atoms, more preferably, an acylamino group having 1to 20 total carbon atoms. For the substituent, a formylamino group, anacetylamino group, a butyrylamino group, a lauroylamino group, abenzoylamino group, a toluoylamino group, a phenoxyacetyl group, a(4-methoxyphenoxy)acetyl group, a 2′,4′-dichlorobenzoylamino group, a2′,4′-di-t-amylbenzoylamino group, an acetylmethylamino group, abenzoylmethylamino group and an acetylbenzylamino group are preferable,and an acetylamino group, a butyrylamino group, a benzoylamino group, atoluoylamino group, a phenoxyacetyl group, a 2′,4′-di-t-amylbenzoylaminogroup, an acetylmethylamino group, a benzoylmethylamino group and anacetylbenzylamino group are more preferable.

The carbamoyl group for R³ or R⁴ is preferably a carbamoyl group having1 to 30 total carbon atoms, more preferably, a carbamoyl group having 1to 20 total carbon atoms. For the substituent, a carbamoyl group, anN-phenylcarbamoyl group, an N-butylcarbamoyl group, an N-octylcarbamoylgroup, an N,N-dimethylcarbamoyl group, an N,N-diethylcarbamoyl group, anN,N-dibutylcarbamoyl group, an N,N-dihexylcarbamoyl group, anN,N-diphenylcarbamoyl group, an N-methyl-N-phenylcarbamoyl group, anN-ethyl-N-phenylcarbamoyl group, an N-methyl-N-tolylcarbamoyl group, amorpholinocarbonyl group, a piperidinocarbonyl group and anN,N-bis(2-methoxyethyl)carbamoyl group are preferable, and anN-butylcarbamoyl group, an N-octylcarbamoyl group, an N-phenylcarbamoylgroup, an N,N-diethylcarbamoyl group, an N,N-dibutylcarbamoyl group andan N-methyl-N-phenylcarbamoyl group are more preferable.

The alkylsulfonyl group for R³ or R⁴ is preferably an alkylsulfonylgroup having 1 to 30 total carbon atoms, more preferably, analkylsulfonyl group having 1 to 20 total carbon atoms. For thesubstituent, a methylsulfonyl group, an ethylsulfonyl group, abutylsulfonyl group, a hexylsulfonyl group and a benzylsulfonyl groupare preferable, and a methylsulfonyl group and a benzylsulfonyl groupare more preferable.

The arylsulfonyl group for R³ or R⁴ is preferably an arylsulfonyl grouphaving 1 to 30 total carbon atoms, more preferably, an arylsulfonylgroup having 1 to 20 total carbon atoms. For the substituent, aphenylsulfonyl group, a 4-methylphenylsulfonyl group, a naphthylsulfonylgroup, a 4-methoxysulfonyl group and a 4-chlorophenyl sulfonylgroup arepreferable, and a methyl sulfonyl group, a phenyl sulfonyl group and a4-methylphenyl sulfonyl group are more preferable.

The acyl group for R³ or R⁴ is preferably an acyl group having 1 to 30total carbon atoms, more preferably, an acyl group having 1 to 20 totalcarbon atoms. For the substituent, a formyl group, an acetyl group, apropionyl group, a pivaloyl group, a butyloyl group, a 4-phenoxybutyloyl group, a benzoyl group, a (4-ethoxyphenyl) carbonyl group, a(2-buthoxyphenyl) carbonyl group and a (4-chlorophenyl) carbonyl groupare preferable, and a formyl group, an acetyl group, a propionyl group,a pivaloyl group, a benzoyl group and a (4-chlorophenyl) carbonyl groupare more preferable.

The sulfamoyl group for R³ or R⁴ is preferably a sulfamoyl group having1 to 30 total carbon atoms, more preferably, a sulfamoyl group having 1to 20 total carbon atoms. For the substituent, a sulfamoyl group, anN-phenyl sulfamoyl group, an N,N-dimethyl sulfamoyl group, anN,N-diethyl sulfamoyl group, an N,N-dibutyl sulfamoyl group, anN,N-dihexyl sulfamoyl group, an N,N-diphenyl sulfamoyl group, anN-methyl-N-phenyl sulfamoyl group, an N-ethyl-N-phenyl sulfamoyl group,an N-methyl-N-tolyl sulfamoyl group, a morpholino sulfonyl group, apiperidino sulfonyl group and an N,N-bis(2-methoxyethyl) sulfonyl groupare preferable, and a sulfamoyl group, an N-phenyl sulfamoyl group, anN,N-dibutyl sulfamoyl group, an N,N-diphenyl sulfamoyl group and anN-methyl-N-phenyl sulfamoyl group are more preferable.

In addition, the polymerizable compound of the present invention ispreferably represented by the following formula (2).

In formula (2), R³ and R⁴ represent a monovalent substituent. R³ and R⁴may be bound to each other to form a ring structure. Preferable examplesof the substituent are the same as those described above in formula (1).

Preferable specific examples of the compound represented by formula (1)or formula (2) will be given below. The present invention is not limitedto these examples.

<Exemplary Compounds>

(Polymer)

The polymer of the present invention is a polymer that comprises acopolymer unit derived from the polymerizable compound (monomer)represented by the above formula (1) or formula (2).

The polymer is preferably a graft copolymer that contains, as acopolymer unit, a polymerizable oligomer having an ethylenicallyunsaturated double bond at its terminal.

Such a polymerizable oligomer having an ethylenically unsaturated doublebond at its terminal is a compound having a given molecular mass and istherefore called a macromonomer.

The polymerizable oligomer contains a polymer chain moiety and apolymerizable functional group moiety at a terminal of the polymerchain. The polymerizable functional group moiety has an ethylenicallyunsaturated double bond. From the viewpoint of obtaining the desiredgraft copolymer, the group having an ethylenically unsaturated doublebond is preferably present at only one of the terminals of the polymerchain. The group having an ethylenically unsaturated double bond ispreferably a (meth)acryloyl group or a vinyl group, and more preferablya (meth)acryloyl group.

The polystyrene-equivalent number-average molecular mass (Mn) of themacromonomer is preferably in the range of 1,000 to 10,000, morepreferably in the range of 2,000 to 9,000.

The polymer chain moiety is generally a homopolymer or copolymer formedfrom at least one monomer selected from the group consisting of alkyl(meth)acrylates, styrene and derivatives thereof, acrylonitrile, vinylacetate, and butadiene, or is polyethylene oxide, polypropylene oxide,and polycaprolactone.

The polymerizable oligomer is preferably an oligomer represented by thefollowing formula (3).

In the formula (3), R¹¹ and R¹³ each independently represent a hydrogenatom or a methyl group. R¹² represents an alkylene group having 1 to 12carbon atoms (preferably an alkylene group having 2 to 4 carbon atoms,may have a substituent (for example, a hydroxyl group) and may be boundto each other via e.g. an ester bond, ether bond or amide bond). Yrepresents a phenyl group, a phenyl group with an alkyl group having 1to 4 carbon atoms, or —COOR¹⁴ (where R¹⁴ represents an alkyl grouphaving 1 to 6 carbon atoms, a phenyl group, or an arylalkyl group having7 to 10 carbon atoms), and q is from 20 to 200. Y is preferably a phenylgroup or —COOR¹⁴ (where R¹⁴ is an alkyl group having 1 to 12 carbonatoms.).

Preferable examples of the polymerizable oligomer (macromonomer) includepolymers in which a (meth)acryloyl group is bound to one terminal ofpolymethyl (meth)acrylate, poly-n-butyl (meth)acrylate, poly-1-butyl(meth)acrylate or polystyrene. Examples of commercially availablepolymerizable oligomers include a polystyrene oligomer having amethacryloyl group at one terminal (Mn=6,000, trade name: AS-6,manufactured by Toagosei Co., Ltd.), a polymethyl methacrylate oligomerhaving a methacryloyl group at one terminal (Mn=6,000, trade name: AA-6,manufactured by Toagosei Co., Ltd.), and a poly-n-butylacrylate oligomerhaving a methacryloyl group at one terminal (Mn=6,000, trade name: AB-6,manufactured by Toagosei Co., Ltd.).

The polymerizable oligomer may be not only polymerizable oligomersrepresented by the formula (3) but also polymerizable oligomersrepresented by the following formula (4).

In the above formula (4), R²¹ represents a hydrogen atom or a methylgroup, and R²² represents an alkylene group having 1 to 8 carbon atoms.X²¹ represents —OR²³ or —COR²⁴, where R²³ and R²⁴ represent a hydrogenatom, an alkyl group or an aryl group. n denotes a number from 2 to 200.

In the formula (4), R²¹ represents a hydrogen atom or a methyl group.R²² represents an alkylene group having 1 to 8 carbon atoms, ispreferably an alkylene group having 1 to 6 carbon atoms and is morepreferably an alkylene group having 2 to 3 carbon atoms. X²¹ represents—OR²³ or —OCOR²⁴, where R²³ represents a hydrogen atom, an alkyl grouphaving 1 to 18 carbon atoms, a phenyl group or a phenyl groupsubstituted with an alkyl group having 1 to 18 carbon atoms. R²⁴represents an alkyl group having 1 to 18 carbon atoms. Also, n denotes anumber 2 to 200, preferably 5 to 100 and more preferably 10 to 100.

Examples of the polymerizable oligomer represented by the formula (4)include polyethylene glycol mono(meth)acrylate, polypropylene glycolmono(meth)acrylate, polyethylene glycol polypropylene glycolmono(meth)acrylate and polytetramethylene glycol monomethacrylate. Thesematerials may be commercially available products or may be thosesynthesized properly.

The polymerizable monomers represented by the formula (4) arecommercially available. Examples of these commercially availableproducts include methoxypolyethylene glycol methacrylate (trade name: NKESTER M-40G, M-90G and M-230G (manufactured by Toagosei Co., Ltd.);trade name: BLENMER-PME-100, PME-200, PME400, PME-1000, PME-2000 andPME4000 (manufactured by NOF CORPORATION)), polyethylene glycolmonomethacrylate (trade name: BLENMER-PE-90, PE-200 and PE-350,manufactured by NOF CORPORATION), polypropylene glycol monomethacrylate(trade name: BLENMER-PP-500, PP-800 and PP-1000, manufactured by NOFCORPORATION), polyethylene glycol polypropylene glycol monomethacrylate(trade name: BLENMER-70PEP-370B, manufactured by NOF CORPORATION),polyethylene glycol polytetramethylene glycol monomethacrylate (tradename: BLENMER-55PET-800, manufactured by NOF CORPORATION) andpolypropylene glycol polytetramethylene glycol monomethacrylate (tradename: BLENMER-NHK-5050, manufactured by NOF CORPORATION).

Further, the polymer for use in the present invention may be a copolymerwith a monomer having a nitrogen atom. Examples of the monomer having anitrogen atom include N,N-dimethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate,1-(N,N-dimethylamino)-1,1-dimethylmethyl(meth)acrylate,N,N-dimethylaminohexyl(meth)acrylate,N,N-diethylaminoethyl(meth)acrylate,N,N-diisopropylaminoethyl(meth)acrylate,N,N-di-n-butylaminoethyl(meth)acrylate,N,N-di-i-butylaminoethyl(meth)acrylate, morpholinoethyl(meth)acrylate,piperidinoethyl(meth)acrylate, 1-pyrrolidinoethyl(meth)acrylate,N,N-methyl-2-pyrrolidylaminoethyl(meth)acrylate andN,N-methylphenylaminoethyl(meth)acrylate (the monomers mentioned aboveare (meth)acrylates); dimethyl(meth)acrylamide, diethyl(meth)acrylamide,diisopropyl (meth)acrylamide, di-n-butyl(meth)acrylamide,di-i-butyl(meth)acrylamide, morpholino(meth)acrylamide,piperidino(meth)acrylamide, N-methyl-2-pyrrolidyl(meth)acrylamide andN,N-methylphenyl(meth)acrylamide (the monomers mentioned above are(meth)acrylamides); 2-(N,N-dimethylamino)ethyl(meth)acrylamide,2-(N,N-diethylamino)ethyl(meth)acrylamide,3-(N,N-diethylamino)propyl(meth)acrylamide,3-(N,N-dimethylamino)propyl(meth)acrylamide,1-(N,N-dimethylamino)-1,1-dimethylmethyl(meth)acrylamide and6(N,N-diethylamino)hexyl(meth)acrylamide (the monomers mentioned aboveare aminoalkyl(meth)acrylamides); p-vinylbenzyl-N,N-dimethylamine,p-vinylbenzyl-N,N-diethylamine, and p-vinylbenzyl-N,N-dihexylamine (themonomers mentioned above are vinylbenzylamines); and 2-vinylpyridine,4-vinylpyridine, and N-vinylimidazole. Among these,N,N-dimethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate,3-(N,N-diethylamino)propyl(meth)acrylamide,3-(N,N-dimethylamino)propyl(meth)acrylamide, 2-vinylpyridine,4-vinylpyridine, and N-vinylimidazole are preferable.

Further, the polymer for use in the present invention may be a copolymerwith other monomers copolymerizable with these polymers. Examples of theother monomers copolymerizable with these polymers may includeunsaturated carboxylic acids (for example, (meth)acrylic acids, crotonicacid, itaconic acid, maleic acid and fumaric acid), aromatic vinylcompounds (for example, styrene, α-methylstyrene, vinyltoluene,2-vinylpyridine, 4-vinylpyridine and N-vinylimidazole),alkyl(meth)acrylates (for example, methyl(meth)acrylate,ethyl(meth)acrylate, n-butyl(meth)acrylate and i-butyl(meth)acrylate),alkylaryl(meth)acrylates (for example, benzyl(meth)acrylate),substituted alkyl(meth)acrylates (for example, glycidyl(meth)acrylateand 2-hydroxyethyl(meth)acrylate), vinyl carboxylates (for example,vinyl acetate and vinyl propionate), vinyl cyanates (for example,(meth)acrylonitrile and α-chloroacrylonitrile) and aliphatic conjugatedienes (for example, 1,3-butadiene and isoprene). Among these compounds,unsaturated carboxylic acids, alkyl(meth)acrylates,alkylaryl(meth)acrylates, vinyl carboxylates and aromatic vinylcompounds are preferable.

The polymer of the present invention is preferably a copolymer thatcomprises a copolymer unit derived from the polymerizable compoundrepresented by the above formula (1) or (2) and a repeating unit givenfrom the polymerizable oligomer (macromonomer), or a copolymer thatcomprises a repeating unit represented by the above formula (1), arepeating unit given from the polymerizable oligomer (macromonomer) anda repeating unit given from a monomer having a nitrogen atom. The abovecopolymer preferably contains the repeating unit represented by theformula (1) in a ratio 5% by weight to 70% by weight (particularly, 5%by weight to 30% by weight) based on all repeating units. In addition,the above copolymer preferably contains the repeating unit given fromthe polymerizable oligomer (macromonomer) in a ratio 30% by weight to95% by weight (particularly, 50% by weight to 90% by weight) based onall repeating units. The above copolymer preferably contains therepeating unit derived from the above monomer containing nitrogencontaining group in a ratio 5% by weight to 80% by weight (particularly,5% by weight to 50% by weight) based on all repeating units.

When the additional monomer copolymerizable with these repeating unitsis used, the quantity of the repeating units derived from the additionalmonomer is preferably in the range of 5% by weight to 30% by weightbased on the total quantity of the repeating units in the copolymer. Theweight-average molecular mass (Mw) of the copolymer is preferably in therange of 1,000 to 200,000, more preferably in the range of 10,000 to100,000. This weight-average molecular mass is a polystyrene equivalentweight-average molecular mass determined by gel permeationchromatography (carrier: tetrahydrofuran).

The structure of the copolymer can be analyzed, for example, byperforming 1H-NMR measurement (a measurement using Mercury (300 MHz)manufactured by Varian, Inc.).

Examples of the graft copolymer that can be suitably used for thepolymer of the present invention will be shown below; however, theseexamples are not intended to limit the present invention.

1) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, and polymethylmethacrylate having a methacryloyl group at itsterminal

2) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, and polyethylene glycol mono(meth)acrylate

3) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, and polycaprolactone having a methacryloyl group at itsterminal

4) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, and polybutyl acrylate having a methacryloyl group at itsterminal

5) A copolymer of the monomer, represented by M-3 of the exemplarycompounds, and polymethylmethacrylate having a methacryloyl group at itsterminal

6) A copolymer of the monomer, represented by M-4 of the exemplarycompounds, and polymethylmethacrylate having a methacryloyl group at itsterminal

7) A copolymer of the monomer, represented by M-5 of the exemplarycompounds, and polyethylene glycol mono(meth)acrylate

8) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 3-(N,N-dimethylamino)propyl acrylamide, andpolymethylmethacrylate having a methacryloyl group at its terminal

9) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, p-vinylbenzyl-N,N-dimethylamine, and polymethylmethacrylatehaving a methacryloyl group at its terminal

10) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 3-(N,N-dimethylamino)propyl acrylamide, and polybutylacrylate having a methacryloyl group at its terminal

11) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 3-(N,N-dimethylamino)ethyl (meth)acrylate, andpolymethylmethacrylate having a methacryloyl group at its terminal

12) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 2-(N,N-dimethylamino)ethyl (meth)acrylate, andpolymethylmethacrylate having a methacryloyl group at its terminal

13) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 3-(N,N-dimethylamino)propyl acrylamide,polymethylmethacrylate having a methacryloyl group at its terminal, andpolyethylene glycol mono(meth)acrylate

14) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, N,N-dimethylacrylamide, and polymethylmethacrylate having amethacryloyl group at its terminal

15) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, methyl (meth)acrylate, and polymethylmethacrylate having amethacryloyl group at its terminal

16) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, methacrylic acid, and polymethylmethacrylate having amethacryloyl group at its terminal

17) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, cyclohexyl (meth)acrylate, and polymethylmethacrylate havinga methacryloyl group at its terminal

18) A copolymer of the monomer, represented by M-1 of the exemplarycompounds, 2-hydroxyethyl (meth)acrylate, and polymethylmethacrylatehaving a methacryloyl group at its terminal

19) A copolymer of the monomer, represented by M-7 of the exemplarycompounds, 3-(N,N-dimethylamino)propyl acrylamide, andpolymethylmethacrylate having a methacryloyl group at its terminal

20) A copolymer of the monomer, represented by M-12 of the exemplarycompounds, 3-(N,N-dimethylamino)propyl acrylamide, andpolymethylmethacrylate having a methacryloyl group at its terminal

21) A copolymer of the monomer, represented by M-13 of the exemplarycompounds, 3-(N,N-dimethylamino)ethyl (meth)acrylate, andpolymethylmethacrylate having a methacryloyl group at its terminal

Such graft copolymers can be obtained by radical polymerization, in asolvent, of the polymerizable oligomer and, optionally, the monomerhaving a nitrogen-containing group and/or other additional monomers. Inthis polymerization, a radical polymerization initiator is used ingeneral. In addition to the initiator, a chain transfer agent (e.g.,2-mercaptoethanol and dodecyl mercaptan) may be further added for thesynthesis of the graft copolymer.

(Ink Composition)

The ink composition of the present invention comprises, in addition tothe polymer, at least (a) other polymerizable compound and (b) apigment, and may comprise other components as necessary.

In the ink composition of the present invention, the polymers of thepresent invention may be used alone, or two or more may be used incombination. The content of the polymer in the ink composition ispreferably 1% by mass to 100% by mass, more preferably 5% by mass to 50%by mass based on the amount of pigment to be added.

In the ink composition of the present invention, a known pigmentdispersant may be used together in addition to the polymer of thepresent invention insofar as the effect of the present invention is notimpaired. The amount of the known pigment dispersant to be added ispreferably 50% by mass or less based on the polymer of the presentinvention.

It is preferable that the polymer of the present invention is used as apigment dispersant. The structure having high affinity to a pigment dueto a van-der-waals interaction ensures good adsorbing ability withrespect to the pigment, making it possible to obtain a stabledispersant. Since the polymer is a high-molecular compound having aspecified repeating structure, the steric repulsive effect of ahigh-molecular chain enables high dispersion stability.

Specific examples of the pigment include organic pigments such as aphthalocyanine base, insoluble azo base, azo lake base, anthraquinonebase, quinacridone base, dioxazine base, diketopyrrolopyrrole base,anthrapyrimidine base, anthanthrone base, indanthrone base, flavanthronebase, perinone base, perylene base, thioindigo base and quinophthalonebase.

The ink composition of the present invention is cured through thefunction of other polymerizable compound (a) when some energy is appliedthereto. Preferably, the ink composition of the present inventioncontains (c) a polymerization initiator and is cured by irradiation withan active energy ray. The active energy ray is not particularly limitedas long as it can impart energy which can generate initiating species inthe ink composition by irradiation therewith. The energy ray include awide range of energy rays such as α-rays, γ-rays, X-rays, ultravioletrays, visible rays and electron rays. Among these energy rays,ultraviolet rays and electron rays are preferable, and ultraviolet raysare more preferable, from the viewpoint of curing sensitivity and theavailability of equipment. Therefore, the ink composition of the presentinvention is preferably an ink composition which can be cured byirradiation with ultraviolet rays as the radiation ray.

<(a) Other Polymerizable Compound>

The curable ink composition of the present invention comprises otherpolymerizable compound (a). Any polymerizable compound may be used asthe other polymerizable compound (a) without any particular limitationas long as it causes a polymerization reaction and is cured by supplyingsome energy. Though any of a monomer, oligomer and polymer may be used,in particular, various known polymerizable monomers known as aphoto-cationic polymerizable monomer or photo-radical polymerizablemonomer are preferable which causes a polymerization reaction byinitiator species generated from the polymerization initiator (c) whichis added as desired.

The polymerizable compounds may be used alone or two or more may be usedin combination with the intention of adjusting, for example, reactionspeed, ink properties and the properties of a cured film. Also, thepolymerizable compounds may be either a monofunctional compound or amultifunctional compound.

Examples of cation-polymerizable monomers that can be used as the otherpolymerizable compound (a) include the epoxy compounds, vinyl ethercompounds, and oxetane compounds described in JP-A Nos. 6-9714,2001-31892, 2001-40068, 2001-55507, 2001-310938, 2001-310937 and2001-220526.

Examples of monofunctional epoxy compounds that can be used as thecation-polymerizable monomer include phenyl glycidyl ether,p-tert-butylphenyl glycidyl ether, butyl glycidyl ether, 2-ethylhexylglycidyl ether, allyl glycidyl ether, 1,2-butylene oxide, 1,3-butadienemonoxide, 1,2-epoxydodecane, epichlorohydrin, 1,2-epoxydecane, styreneoxide, cyclohexene oxide, 3-methacryloyloxymethylcyclohexene oxide,3-acryloyloxymethylcyclohexene oxide, and 3-vinylcyclohexene oxide.

Examples of polyfunctional epoxy compounds that can be used as thecation-polymerizable monomer include bisphenol A diglycidyl ether,bisphenol F diglycidyl ether, bisphenol S diglycidyl ether, brominatedbisphenol A diglycidyl ether, brominated bisphenol F diglycidyl ether,brominated bisphenol S diglycidyl ether, epoxy novolak resin,hydrogenated bisphenol A diglycidyl ether, hydrogenated bisphenol Fdiglycidyl ether, hydrogenated bisphenol S diglycidyl ether,3,4-epoxycyclohexyl methyl-3′,4′-epoxycyclohexane carboxylate,2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-metha-dioxane,bis(3,4-epoxycyclohexylmethyl)adipate, vinylcyclohexene oxide,4-vinylepoxycyclohexane, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, 3,4-epoxy-6-methylcyclohexyl-3′,4′-epoxy-6′-methylcyclohexanecarboxylate, methylenebis(3,4-epoxycyclohexane), dicylopentadienediepoxide, ethyleneglycol di(3,4-epoxycyclohexylmethyl)ether,ethylenebis(3,4-epoxycyclohexane carboxylate), dioctylepoxyhexahydrophthalate, di-2-ethylhexyl epoxyhexahydrophthalate,1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether,glycerin triglycidyl ether, trimethylolpropane triglycidyl ether,polyethylene glycol diglycidyl ether, polypropylene glycol diglycidylethers, 1,1,3-tetradecadiene dioxide, limonene dioxide, 1,2,7,8-diepoxyoctane, and 1,2,5,6-diepoxy cyclooctane.

Among these epoxy compounds, aromatic epoxides and alicyclic epoxidesare preferable since they are advantageous in respect of the curingrate, alicyclic epoxides are more preferable.

Examples of monofunctional vinyl ether compounds that can be used as thecation-polymerizable monomer include methyl vinyl ether, ethyl vinylether, propyl vinyl ether, n-butyl vinyl ether, t-butyl vinyl ether,2-ethylhexyl vinyl ether, n-nonyl vinyl ether, lauryl vinyl ether,cyclohexyl vinyl ether, cyclohexyl methyl vinyl ether,4-methylcyclohexylmethyl vinyl ether, benzyl vinyl ether,dicyclopentenyl vinyl ether, 2-dicyclopentenoxy ethyl vinyl ether,methoxyethyl vinyl ether, ethoxyethyl vinyl ether, butoxyethyl vinylether, methoxyethoxyethyl vinyl ether, ethoxyethoxyethyl vinyl ether,methoxy polyethylene glycol vinyl ether, tetrahydrofurfuryl vinyl ether,2-hydroxyethyl vinyl ether, 2-hydroxypropyl vinyl ether, 4-hydroxybutylvinyl ether, 4-hydroxymethyl cyclohexylmethyl vinyl ether, diethyleneglycol monovinyl ether, polyethylene glycol vinyl ether, chloroethylvinyl ether, chlorobutyl vinyl ether, chloroethoxyethyl vinyl ether,phenylethyl vinyl ether, and phenoxy polyethylene glycol vinyl ether.

Examples of multifunctional vinyl ether compounds that can be used asthe cation-polymerizable monomer include: divinyl ethers such asethylene glycol divinyl ether, diethylene glycol divinyl ether,triethylene glycol divinyl ether, polyethylene glycol divinyl ether,propylene glycol divinyl ether, butylene glycol divinyl ether, hexanediol divinyl ether, bisphenol A alkylene oxide divinyl ether, andbisphenol F alkylene oxide divinyl ether; and multifunctional vinylethers such as trimethylolethane trivinyl ether, trimethylolpropanetrivinyl ether, ditrimethylolpropane tetravinyl ether, glycerin trivinylether, pentaerythritol tetravinyl ether, dipentaerythritol pentavinylether, dipentaerythritol hexavinyl ether, ethylene oxide-addedtrimethylolpropane trivinyl ether, propylene oxide-addedtrimethylolpropane trivinyl ether, ethylene oxide-addedditrimethylolpropane tetravinyl ether, propylene oxide-addedditrimethylolpropane tetravinyl ether, ethylene oxide-addedpentaerythritol tetravinyl ether, propylene oxide-added pentaerythritoltetravinyl ether, ethylene oxide-added dipentaerythritol hexavinylether, and propylene oxide-added dipentaerythritol hexavinyl ether.

Among the multifunctional vinyl ether compounds described above, a di-or trivinyl ether compound is preferable from the viewpoint of curingproperties, adhesion to a recording medium, and the surface hardness ofan image formed, and a divinyl ether compound is particularlypreferable.

The oxetane compound that can be used as the cation-polymerizablemonomer refers to a compound having an oxetane ring. Such an oxetanecompound may be selected arbitrarily from known oxetane compounds, forexample, those described in JP-A Nos. 2001-220526, 2001-310937, and2003-341217. The compound having an oxetane ring is preferably acompound having 1 to 4 oxetane rings in its structure. By using such acompound, the viscosity of the ink composition can be maintained in arange which enables easy handling, and the ink after curing adheresstrongly to the recording medium.

Examples of monofunctional oxetanes that can be used as thecation-polymerizable monomer include 3-ethyl-3-hydroxymethyl oxetane,3-(meth)allyloxymethyl-3-ethyl oxetane,(3-ethyl-3-oxetanylmethoxy)methyl benzene,4-fluoro-[1-(3-ethyl-3-oxetanylmethoxy)methyl]benzene,4-methoxy-[1-(3-ethyl-3-oxetanylmethoxy)methyl]benzene,[1-(3-ethyl-3-oxetanylmethoxy)ethyl]phenyl ether,isobutoxymethyl(3-ethyl-3-oxetanylmethyl)ether,isobornyloxyethyl(3-ethyl-3-oxetanylmethyl)ether,isobornyl(3-ethyl-3-oxetanylmethyl)ether,2-ethylhexyl(3-ethyl-3-oxetanylmethyl)ether, ethyldiethyleneglycol(3-ethyl-3-oxetanylmethyl)ether, dicyclopentadiene(3-ethyl-3-oxetanylmethyl)ether, dicyclopentenyloxyethyl(3-ethyl-3-oxetanylmethyl)ether, dicyclopentenyl(3-ethyl-3-oxetanylmethyl)ether, tetrahydrofurfuryl(3-ethyl-3-oxetanylmethyl)ether, tetrabromophenyl(3-ethyl-3-oxetanylmethyl)ether, 2-tetrabromophenoxyethyl(3-ethyl-3-oxetanylmethyl)ether, tribromophenyl(3-ethyl-3-oxetanylmethyl)ether, 2-tribromophenoxyethyl(3-ethyl-3-oxetanylmethyl)ether, 2-hydroxyethyl(3-ethyl-3-oxetanylmethyl)ether, 2-hydroxypropyl(3-ethyl-3-oxetanylmethyl)ether, butoxyethyl(3-ethyl-3-oxetanylmethyl)ether, pentachlorophenyl(3-ethyl-3-oxetanylmethyl)ether, pentabromophenyl(3-ethyl-3-oxetanylmethyl)ether, andbornyl(3-ethyl-3-oxetanylmethyl)ether.

Examples of multifunctional oxetanes that can be used as thecation-polymerizable monomer include 3,7-bis(3-oxetanyl)-5-oxa-nonane,3,3′-(1,3-(2-methylenyl) propanediylbis (oxymethylene))bis-(3-ethyloxetane), 1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene,1,2-bis[(3-ethyl-3-oxetanylmethoxy)methyl]ethane,1,3-bis[(3-ethyl-3-oxetanylmethoxy)methyl]propane, ethylene glycolbis(3-ethyl-3-oxetanylmethyl)ether, dicyclopentenylbis(3-ethyl-3-oxetanylmethyl)ether, triethylene glycolbis(3-ethyl-3-oxetanylmethyl)ether, tetraethylene glycolbis(3-ethyl-3-oxetanylmethyl)ether, tricyclodecanediyldimethylene(3-ethyl-3-oxetanylmethyl)ether, trimethylolpropanetris(3-ethyl-3-oxetanylmethyl)ether, 1,4-bis(3-ethyl-3-oxetanylmethoxy)butane, 1,6-bis(3-ethyl-3-oxetanylmethoxy) hexane, pentaerythritoltris(3-ethyl-3-oxetanylmethyl)ether, pentaerythritoltetrakis(3-ethyl-3-oxetanylmethyl)ether, polyethylene glycolbis(3-ethyl-3-oxetanylmethyl)ether, dipentaerythritolhexakis(3-ethyl-3-oxetanylmethyl)ether, dipentaerythritolpentakis(3-ethyl-3-oxetanylmethyl)ether, dipentaerythritoltetrakis(3-ethyl-3-oxetanylmethyl)ether, caprolactone-modifieddipentaerythritol hexakis(3-ethyl-3-oxetanylmethyl)ether,caprolactone-modified dipentaerythritolpentakis(3-ethyl-3-oxetanylmethyl)ether, ditrimethylolpropanetetrakis(3-ethyl-3-oxetanylmethyl)ether, EO-modified bisphenol Abis(3-ethyl-3-oxetanylmethyl)ether, PO-modified bisphenol Abis(3-ethyl-3-oxetanylmethyl)ether, EO-modified hydrogenated bisphenol Abis(3-ethyl-3-oxetanylmethyl)ether, PO-modified hydrogenated bisphenol Abis(3-ethyl-3-oxetanylmethyl)ether, and EO-modified bisphenol F(3-ethyl-3-oxetanylmethyl)ether.

Such compounds having an oxetane ring are described in detail in columns[0021] to [0084] of JP-A 2003-341217 mentioned above, and the compoundsdescribed therein can be suitably used in the present invention as well.

Among the oxetane compounds as the cation-polymerizable monomer, acompound having one or two oxetane rings is preferable from theviewpoint of the viscosity and adhesiveness of the ink composition.

In the ink composition of the present invention, as acation-polymerizable monomer, only one compound may be used, or two ormore compounds may be used in combination. From the viewpoint ofeffective suppression of shrinkage upon curing, it is preferable to usea combination of at least one oxetane compound and at least one compoundselected from epoxy compounds and vinyl ether compounds.

In the present invention, the other polymerizable compound (a) may beselected from a wide variety of known radical polymerizable monomersthat undergo polymerization reaction in the presence of an initiatorspecies generated from a photoradical initiator.

Examples of such radical polymerizable monomers include (meth)acrylates,(meth)acrylamides, and aromatic vinyls. In the specification, the term“(meth)acrylate” is occasionally used to mean “acrylate” and/or“methacrylate”, and the term “(meth)acryl” is occasionally used to mean“acryl” and/or “methacryl”.

Examples of (meth)acrylates that can be used as the radicalpolymerizable monomer include monofunctional (meth)acrylates,bifunctional (meth)acrylates, trifunctional (meth)acrylates,tetrafunctional (meth)acrylates, pentafunctional (meth)acrylates, andhexafunctional (meth)acrylates.

Examples of monofunctional (meth)acrylates include hexyl (meth)acrylate,2-ethylhexyl (meth)acrylate, tert-octyl (meth)acrylate, isoamyl(meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, stearyl(meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate,4-n-butylcyclohexyl (meth)acrylate, bornyl (meth)acrylate, isobornyl(meth)acrylate, benzyl (meth)acrylate, 2-ethylhexyldiglycol(meth)acrylate, butoxyethyl (meth)acrylate, 2-chloroethyl(meth)acrylate, 4-bromobutyl (meth)acrylate, cyanoethyl (meth)acrylate,benzyl (meth)acrylate, butoxymethyl (meth)acrylate, 3-methoxybutyl(meth)acrylate, alkoxymethyl (meth)acrylate, alkoxyethyl (meth)acrylate,2-(2-methoxyethoxy)ethyl (meth)acrylate, 2-(2-butoxyethoxy)ethyl(meth)acrylate, 2,2,2-tetrafluoroethyl (meth)acrylate,1H,1H,2H,2H-perfluorodecyl (meth)acrylate, 4-butylphenyl (meth)acrylate,phenyl (meth)acrylate, 2,4,5-tetramethylphenyl (meth)acrylate,4-chlorophenyl (meth)acrylate, phenoxymethyl (meth)acrylate,phenoxyethyl (meth)acrylate, glycidyl (meth)acrylate, glycidyloxybutyl(meth)acrylate, glycidyloxyethyl (meth)acrylate, glycidyloxypropyl(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, hydroxyalkyl(meth)acrylate, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl(meth)acrylate, 4-hydroxybutyl (meth)acrylate, 3-hydroxybutyl(meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl(meth)acrylate, dimethylaminopropyl (meth)acrylate, diethylaminopropyl(meth)acrylate, trimethoxysilylpropyl (meth)acrylate,trimethylsilylpropyl (meth)acrylate, polyethylene oxide monomethyl ether(meth)acrylate, oligoethylene oxide monomethyl ether (meth)acrylate,polyethylene oxide (meth)acrylate, oligoethylene oxide (meth)acrylate,oligoethylene oxide monoalkyl ether (meth)acrylate, polyethylene oxidemonoalkyl ether (meth)acrylate, dipropylene glycol (meth)acrylate,polypropylene oxide monoalkyl ether (meth)acrylate, oligopropylene oxidemonoalkyl ether (meth)acrylate, 2-methacryloyloxytylsuccinic acid,2-methacryloyloxyhexahydrophthalic acid,2-methacryloyloxyethyl-2-hydroxypropyl phthalate, butoxydiethyleneglycol (meth)acrylate, trifluoroethyl (meth)acrylate,perfluorooctylethyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, EO-modified phenol (meth)acrylate, EO-modified cresol(meth)acrylate, EO-modified nonyl phenol (meth)acrylate, PO-modifiednonyl phenol (meth)acrylate, EO-modified 2-ethylhexyl (meth)acrylate,dicyclopentenyl (meth)acrylate, and dicyclopentanyl (meth)acrylate.

Examples of bifunctional (meth)acrylates include 1,6-hexanedioldi(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycoldi(meth)acrylate, 2,4-dimethyl-1,5-pentanediol di(meth)acrylate,butylethylpropanediol (meth)acrylate, ethoxylated cyclohexane methanoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, oligoethyleneglycol di(meth)acrylate, ethylene glycol di(meth)acrylate,2-ethyl-2-butyl-butanediol di(meth)acrylate, hydroxypivalic acidneopentyl glycol di(meth)acrylate, EO-modified bisphenol Adi(meth)acrylate, bisphenol F polyethoxy di(meth)acrylate, polypropyleneglycol di(meth)acrylate, oligopropylene glycol di(meth)acrylate,1,4-butanediol di(meth)acrylate, 2-ethyl-2-butylpropanedioldi(meth)acrylate, 1,9-nonane di(meth)acrylate, propoxylated ethoxylatedbisphenol A di(meth)acrylate, tricyclodecane di(meth)acrylate, neopentylglycol propyleneoxy diacrylate, dipropylene glycol di(meth)acrylate, andpropoxylated neopentyl glycol di(meth)acrylate.

Examples of trifunctional (meth)acrylates include trimethylolpropanetri(meth)acrylate, trimethylolethane tri(meth)acrylate,trimethylolpropane alkylene oxide-modified tri(meth)acrylate,pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate,trimethylolpropane tri((meth)acryloyloxypropyl)ether, isocyanuric acidalkylene oxide-modified tri(meth)acrylate, propionic aciddipentaerythritol tri(meth)acrylate, tri((meth)acryloyloxyethyl)isocyanurate, hydroxypival aldehyde-modified dimethylolpropanetri(meth)acrylate, sorbitol tri(meth)acrylate, propoxylatedtrimethylolpropane tri(meth)acrylate, and ethoxylated glycerintriacrylate.

Examples of tetrafunctional (meth)acrylates include pentaerythritoltetra(meth)acrylate, sorbitol tetra(meth)acrylate, ditrimethylolpropanetetra(meth)acrylate, propionic acid dipentaerythritoltetra(meth)acrylate, and ethoxylated pentaerythritoltetra(meth)acrylate.

Examples of pentafunctional (meth)acrylates include sorbitolpenta(meth)acrylate and dipentaerythritol penta(meth)acrylate.

Examples of hexafunctional (meth)acrylates include dipentaerythritolhexa(meth)acrylate, sorbitol hexa(meth)acrylate, phosphazene alkyleneoxide-modified hexa(meth)acrylate, and caprolactone-modifieddipentaerythritol hexa(meth)acrylate.

Examples of (meth)acrylamides that can be used as the radicalpolymerizable monomer include (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl (meth)acrylamide, N-propyl (meth)acrylamide,N-n-butyl (meth)acrylamide, N-t-butyl (meth)acrylamide, N-butoxymethyl(meth)acrylamide, N-isopropyl (meth)acrylamide, N-methylol(meth)acrylamide, N,N-dimethyl (meth)acrylamide, N,N-diethyl(meth)acrylamide, and (meth)acryloyl morpholine.

Examples of aromatic vinyls that can be used as the radicalpolymerizable monomer include styrene, methyl styrene, dimethyl styrene,trimethyl styrene, ethyl styrene, isopropyl styrene, chloromethylstyrene, methoxy styrene, acetoxy styrene, chlorostyrene,dichlorostyrene, bromostyrene, methyl vinylbenzoate, 3-methyl styrene,4-methyl styrene, 3-ethyl styrene, 4-ethyl styrene, 3-propyl styrene,4-propyl styrene, 3-butyl styrene, 4-butyl styrene, 3-hexyl styrene,4-hexyl styrene, 3-octyl styrene, 4-octyl styrene, 3-(2-ethylhexyl)styrene, 4-(2-ethylhexyl) styrene, allyl styrene, isopropenyl styrene,butenyl styrene, octenyl styrene, 4-t-butoxycarbonyl styrene, 4-methoxystyrene, and 4-t-butoxy styrene.

Examples of the radical polymerizable monomer in the present inventioninclude vinyl esters [vinyl acetate, vinyl propionate, vinyl versateetc.], allyl esters [allyl acetate etc.], halogen-containing monomers[vinylidene chloride, vinyl chloride etc.], vinyl ethers [methyl vinylether, butyl vinyl ether, hexyl vinyl ether, methoxy vinyl ether,2-ethyl hexyl vinyl ether, methoxyethyl vinyl ether, cyclohexyl vinylether, chloroethyl vinyl ether, triethyleneglycol divinyl ether etc.]and vinyl cyanides [(meth)acrylonitrile etc.], and olefins [ethylene,propylene etc.].

Among these, the radical polymerizable monomer in the present inventionis preferably (meth)acrylates or (meth)acrylamides from the viewpoint ofthe curing rate, and more preferably a tetrafunctional orhigher-functional (meth)acrylate from the viewpoint of the curing rate.From the viewpoint of the viscosity of the ink composition, it ispreferable to use a combination of a multifunctional (meth)acrylate suchas described above and a monofunctional or bifunctional (meth)acrylateor (meth)acrylamide.

Appropriate amount of the other polymerizable compound (a) in the inkcomposition is 50% by mass to 95% by mass, based on the total solidcontent of the composition; preferably, the content of the otherpolymerizable compound (a) in the ink composition is 60% by mass to 92%by mass, still preferably 70% by mass to 90% by mass, based on the totalsolid content of the composition

<(b) Pigment>

The ink composition of the present invention contains a pigment as anessential component. Owing to the function of the polymer, fine pigmentparticles with a small particle diameter are dispersed uniformly andstably in the ink composition, which allows the formation of sharpimages excellent in coloring property.

The pigment is not particularly limited and may be appropriatelyselected from a wide variety of known pigments and dyes according to thepurpose. Because the pigment is contained as a colorant, the imagesobtained by using the ink composition of the present invention areexcellent in weather resistance.

The pigment contained in the ink composition of the present invention isnot particularly limited. Organic pigments and inorganic pigments thatare usually available on the market and also, pigments obtained bydyeing resin particles with dyes may be used. In addition, commerciallyavailable pigment dispersants and surface treated pigments, for example,those obtained by dispersing pigments in an insoluble resin as adispersion medium or those obtained by grafting a resin on the surfaceof a pigment may be used insofar as they do not impair the effect of thepresent invention.

Examples of these pigments include the pigments described, for example,in Seijiro Itoh, “Dictionary of Pigments” (2000), W. Herbst and K.Hunger, “Industrial Organic Pigments”, and JP-A Nos. 2002-12607,2002-188025, 2003-26978, and 2003-342503.

Examples of the organic and inorganic pigments contained in the inkcomposition of the present invention include the following. Examples ofpigments of yellow color include: monoazo pigments such as C.I. PigmentYellow 1 (Fast Yellow G, etc.) and C.I. Pigment Yellow 74; disazopigments such as C.I. Pigment Yellow 12 (Disazo Yellow, etc.), C.I.Pigment Yellow 17, C.I. Pigment Yellow 97, C.I. Pigment Yellow 3, C.I.Pigment Yellow 16, C.I. Pigment Yellow 83, C.I. Pigment Yellow 155, andC.I. Pigment Yellow 219; non-benzidine azo pigments such as C.I. PigmentYellow 180; azolake pigments such as C.I. Pigment Yellow 100 (tartrazineyellow lake, etc.); condensation azo pigments such as C.I. PigmentYellow 95 (Condensation Azo Yellow, etc.), C.I. Pigment Yellow 93, C.I.Pigment Yellow 94, C.I. Pigment Yellow 128, and C.I. Pigment Yellow 166;acidic-dye lake pigments such as C.I. Pigment Yellow 115 (quinolineyellow lake, etc.); basic-dye lake pigments such as C.I. Pigment Yellow18 (thioflavin lake, etc.); anthraquinone pigments such as C.I. PigmentYellow 24 (fravantrone yellow, etc.); quinophtharone pigments such asC.I. Pigment Yellow 110 (quinophtharone yellow, etc.); isoindolinepigments such as C.I. Pigment Yellow 139 (isoindoline yellow, etc.);pyrazolone pigments such as C.I. Pigment Yellow 60 (pyrazolone yellow,etc.); acetolone pigments such as C.I. Pigment Yellow 120, C.I. PigmentYellow 154, C.I. Pigment Yellow 167, C.I. Pigment Yellow 151, C.I.Pigment Yellow 175, C.I. Pigment Yellow 180, C.I. Pigment Yellow 181,and C.I. Pigment Yellow 194; metal-complex-salt pigments such as C.I.Pigment Yellow 150; nitroso pigments such as C.I. Pigment Yellow 153(nickel nitroso yellow, etc.); and metal-complex-salt azomethinepigments such as C.I. Pigment Yellow 117 (copper azomethine yellow,etc.).

Examples of pigments of red or magenta color include: monoazo pigmentssuch as C.I. Pigment Red 3 (toluidine red, etc.); B-naphthol pigmentssuch as C.I. Pigment Red 1, C.I. Pigment Red 4, and C.I. Pigment Red 6;disazo pigments such as C.I. Pigment Red 38 (Pyrazolone Red B, etc.);azolake pigments such as C.I. Pigment Red 53:1 (Lake Red C, etc.), C.I.Pigment Red 57:1 (Brilliant Carmine 6B, etc.), C.I. Pigment Red 52:1,and C.I. Pigment Red 48 (B-oxynaphthoic acid Lake, etc.); condensationazo pigments such as C.I. Pigment Red 144, C.I. Pigment Red 166, C.I.Pigment Red 220, C.I. Pigment Red 214, C.I. Pigment Red 221, and C.I.Pigment Red 242 (Condensation Azo Red, etc.); acidic dye lake pigmentssuch as C.I. Pigment Red 174 (Phloxine B Lake, etc.) and C.I. PigmentRed 172 (Erythrosine Lake, etc.); basic dye lake pigments such as C.I.Pigment Red 81 (Rhodamine 6G′ Lake, etc.); anthraquinone pigments suchas C.I. Pigment Red 177 (dianthraquinolyl red, etc.); thioindigopigments such as C.I. Pigment Red 88 (Thioindigo Bordeaux, etc.);perynone pigments such as C.I. Pigment Red 194 (perynone red, etc.);perylene pigments such as C.I. Pigment Red 149, C.I. Pigment Red 179,C.I. Pigment Red 178, C.I. Pigment Red 190, C.I. Pigment Red 224, andC.I. Pigment Red 123; quinacridone pigments such as C.I. Pigment Violet19 (unsubstituted quinacridone), C.I. Pigment Red 122, C.I. Pigment Red262, C.I. Pigment Red 207, and C.I. Pigment Red 209; isoindolinonepigments such as C.I. Pigment Red 180 (Isoindolinone Red 2BLT, etc.);alizarin lake pigments such as C.I. Pigment Red 83 (madder lake, etc.);naphtholone pigments such as C.I. Pigment Red 171, C.I. Pigment Red 175,C.I. Pigment Red 176, C.I. Pigment Red 185, and C.I. Pigment Red 208;naphthol AS lake pigments such as C.I. Pigment Red 247; naphthol ASpigments such as C.I. Pigment Red 2, C.I. Pigment Red 5, C.I. PigmentRed 21, C.I. Pigment Red 170, C.I. Pigment Red 187, C.I. Pigment Red256, C.I. Pigment Red 268, and C.I. Pigment Red 269; anddiketopyrrolopyrrole pigments such as C.I. Pigment Red 254, C.I. PigmentRed 255, C.I. Pigment Red 264, and C.I. Pigment Red 272.

Examples of blue or cyan pigments include disazo pigments such as C.I.Pigment Blue 25 (dianisidine blue, etc.); phthalocyanine pigments suchas C.I. pigment blue 15, C.I. pigment blue 15:1, C.I. pigment blue 15:2,C.I. pigment blue 15:3, C.I. pigment blue 15:4, C.I. pigment blue 15:6,and C.I. pigment blue 16 (phthalocyanine blue, etc.); acidic dye lakepigments such as C.I. pigment blue 24 (peacock blue lake, etc.); basicdye lake pigments such as C.I. Pigment Blue 1 (Victoria Pure Blue BOLake, etc.); anthraquinone pigments such as C.I. pigment blue 60(indanthron blue, etc.); and alkali blue pigments such as C.I. PigmentBlue 18 (alkali blue V-5:1).

Examples of green pigments include phthalocyanine pigments such as C.I.Pigment Green 7 (phthalocyanine green) and C.I. Pigment Green 36(phthalocyanine green); and azo metal complex pigments such as C.I.Pigment Green 8 and C.I. Pigment Green 10.

Examples of orange pigments include isoindoline pigments such as C.I.Pigment Orange 66 (isoindoline orange); anthraquinone pigments such asC.I. Pigment Orange 51 (dichloropyranthron orange); B-naphthol pigmentssuch as C.I. Pigment Orange 2, C.I. Pigment Orange 3, and C.I. PigmentOrange 5; naphthol AS pigments such as C.I. Pigment Orange 4, C.I.Pigment Orange 22, C.I. Pigment Orange 24, C.I. Pigment Orange 38, andC.I. Pigment Orange 74; isoindolinone pigments such as C.I. PigmentOrange 61; perynone pigments such as C.I. Pigment Orange 43; disazopigments such as C.I. Pigment Orange 15 and C.I. Pigment Orange 16;quinacridone pigments such as C.I. Pigment Orange 48 and C.I. PigmentOrange 49; acetolone pigments such as C.I. Pigment Orange 36, C.I.Pigment Orange 62, C.I. Pigment Orange 60, C.I. Pigment Orange 64, andC.I. Pigment Orange 72; and pyrazolone pigments such as C.I. PigmentOrange 13 and C.I. Pigment Orange 34.

Examples of brown pigments include naphtholone pigments such as C.I.Pigment Brown 25 and C.I. Pigment Brown 32.

Examples of violet pigments include naphtholone pigments such as C.I.Pigment Violet 32; perylene pigments such as C.I. Pigment Violet 29;naphthol AS pigments such as C.I. Pigment Violet 13, C.I. Pigment Violet17, and C.I. Pigment Violet 50; and dioxazine pigments such as C.I.Pigment Violet 23 and C.I. Pigment Violet 37.

Examples of black pigments include indazine pigments such as carbonblack, titanium black, and C.I. Pigment Black 1 (aniline black); andperylene pigments such as C.I. Pigment Black 31 and C.I. Pigment Black32.

Examples of white pigments include basic lead carbonate (2PbCO₃Pb(OH)₂,so-called silver white), zinc oxide (ZnO, so-called zinc white),titanium oxide (TiO₂, so-called titanium white), and strontium titanate(SrTiO₃, so-called titanium strontium white). The inorganic particles tobe used for white pigment may be particles of a simple substance, or maybe an oxide of, for example, silicon, aluminum, zirconium, or titanium,or composite particles with an organometallic compound or an organiccompound.

Since titanium oxide has a lower specific gravity and a higherrefractive index than other white pigments and is more stable chemicallyor physically, titanium oxide has a greater masking and coloringpotential as a pigment, and is further excellent in resistance to acidor alkali and other environmental factors. Thus, the use of titaniumoxide as a white pigment is preferable. Of course, other white pigment(including white pigments other than those described above) may be usedas necessary.

For dispersing the pigment, dispersing machines such as a ball mill, asand mill, an attriter, a roll mill, a jet mill, a homogenizer, a paintshaker, a kneader, an agitator, a Henschel mixer, a colloid mill, anultrasonic wave homogenizer, a pearl mill, and a wet jet mill, can beused.

When the pigment is dispersed, it is preferable to add the polymer ofthe present invention.

A synergist suitable for the pigment may be used as a dispersing aid asnecessary. The dispersing aid is preferably added in an amount of 1 partby mass to 50 parts by mass based on 100 parts by mass of the pigment.

In the ink composition, a solvent may be added as the dispersion mediumfor various components such as pigment, or the other polymerizablecompound (a), which is a low-molecular-mass component, may be used as asolvent-free dispersion medium. The ink composition according to thepresent invention is preferably free of solvent because the compositionis preferably a radiation-curable ink that is cured after applied onto arecording medium. If the solvent remains in the cured ink image, solventresistance may be deteriorated and a problem of VOC (Volatile OrganicCompound) may occur. Thus, the dispersion medium is preferably a otherpolymerizable compound (a), particularly preferably a polymerizablecompound having the lowest viscosity, in view of the improvement in thedispersibility and handling property of the ink composition.

A pigment having a smaller diameter is more excellent in coloringproperties. Therefore, the average particle diameter of the pigment tobe used is preferably in the range of about 0.01 μm to 0.4 μm, morepreferably in the range of 0.02 μm to 0.2 μm. The maximum particlediameter may be 3 μm, preferably 1 μm; such a maximum particle diametercan be achieved by appropriate selections of the pigment (b), thedispersant, the dispersing medium, the dispersion conditions, and thefiltration conditions. By controlling the particle diameter, clogging ina head nozzle can be prevented, and the storage stability of ink, thetransparency of ink, and the curing sensitivity can be secured. Becausethe polymer excellent in dispersibility and stability is used in thepresent invention, a uniform and stable dispersion can be obtained evenwhen fine pigment particles are used.

The particle diameter of the pigment in the ink composition can bemeasured by a known measurement method. Specifically, the particlediameter can be measured by a centrifugal-sedimentationlight-transmission method, an X-ray transmission method, a laserdiffraction/scattering method, or a dynamic light scattering method.

When the pigment is an organic pigment, the amount of the pigment in theink composition is preferably 1% by mass to 20% by mass, more preferably2% by mass to 10% by mass, in terms of solid content. When the pigmentis an inorganic pigment, the amount of the pigment in the inkcomposition is preferably 1% by mass to 30% by mass, more preferably 2%by mass to 25% by mass, in terms of solid content.

In the ink composition of the present invention, various additives maybe used additionally according to the purpose, in addition to theabove-described essential components. These optional components aredescribed.

<(c) Polymerization Initiator>

The ink composition of the present invention preferably contains aradical polymerization initiator or a cation polymerization initiatorand more preferably contains a photopolymerization initiator.

The photopolymerization initiator in the present invention is a compoundthat is changed chemically through the action of light or an interactionwith a sensitizing dye put in an electron excited state to produce atleast one of a radical, an acid and a base.

The photopolymerization initiator may be selected appropriately frominitiators having sensitivity to the activated ray for irradiation, suchas UV ray at 400 nm to 200 nm, far UV ray, g-line, h-line, i-line, KrFexcimer laser light, ArF excimer laser light, electron ray, X-ray,molecular beam, or ion beam.

For the photopolymerization initiator, any of common photopolymerizationinitiators known in the art may be used. Examples thereof are described,for example, in Bruce M. Monroe et al., Chemical Revue, 93, 435 (1993);R. S. Davidson, Journal of Photochemistry and biology A: Chemistry, 73,81 (1993); J. P. Faussier, “Photoinitiated Polymerization-Theory andApplications”: Rapra Review vol. 9, Report, Rapra Technology (1998); andM. Tsunooka et al., Prog. Polym. Sci., 21, 1 (1996). Compounds favorablyused in chemical amplification photoresists and for photocationicpolymerization are described in Japanese Research Association forOrganic Electronics Materials Ed., “Organic Materials for Imaging”(published by Bun-Shin Shuppan (1993), pp. 187 to 192), and thosecompounds can be used. The compounds that undergo oxidative or reductivebond cleavage through the interaction with the electronically-excitedstate of sensitizing dye are also known, and described, for example inF. D. Saeva, Topics in Current Chemistry, 156, 59 (1990); G. G. Maslak,Topics in Current Chemistry, 168, 1 (1993); H. B. Shuster et al., JACS,112, 6329 (1990); and I. D. F. Eaton et al., JACS, 102, 3298 (1980).

Examples of such photopolymerization initiators include (i) aromaticketones, (ii) aromatic onium salt compounds, (iii) organic peroxides,(iv) hexaarylbiimidazole compounds, (v) ketoxime ester compounds, (vi)borate compounds, (vii) azinium compounds, (viii) metallocene compounds,(ix) active ester compounds, and (x) compounds containing acarbon-halogen bond.

In addition, acylphosphine oxide may also be used as thephotopolymerization initiator.

For the aromatic ketones (i), the compounds each having a benzophenoneor thioxanthone skeleton described, for example in “Radiation Curing inPolymer Science and Technology” J. P. Fouassier and J. F. Rabek (1993),pp. 77 to 117 are preferable; and α-thio benzophenone compoundsdescribed in Japanese Patent Application Publication (JP-B) No. 47-6416;the benzoin ether compounds described in JP-B No. 47-3981; theα-substituted benzoin compounds described in JP-B No. 47-22326; thebenzoin derivatives described in JP-B No. 47-23664; the aroyl phosphonicacid esters described in JP-A No. 57-30704; the dialkoxybenzophenonesdescribed in JP-B No. 60-26483; the benzoin ethers described in JP-B No.60-26403 and JP-A No. 62-81345; the α-amino benzophenones described inJP-B No. 1-34242, U.S. Pat. No. 4,318,791, and EP Patent No. 0284561A1;p-di(dimethylaminobenzoyl)benzene described in JP-A No. 2-211452; thethio-substituted aromatic ketones described in JP-A No. 61-194062; theacylphosphine sulfides described in JP-B No. 2-9597; the acylphosphinesdescribed in JP-B No. 2-9596; the thioxanthones described in JP-B No.63-61950; the coumarins described in JP-B No. 5942864;monoacylphosphineoxide described in JP-B Nos. 60-8047 and 6340799; andbisacylphosphineoxide described in JP-A Nos. 3-101686, 5-345790 and6-298818 are more preferable.

Examples of the aromatic onium salt compounds (II) include aromaticonium salts of the elements in Groups V, VI and VII in the periodictable, specifically, aromatic onium salts of N, P, As, Sb, Bi, O, S, Se,Te, and I. For example, the iodonium salts described in EP Patent No.104143, U.S. Pat. No. 4,837,124, and JP-A Nos. 2-150848 and 2-96514; thesulfonium salts described in EP Patent Nos. 370693, 233567, 297443,297442, 279210, and 422570 and U.S. Pat. Nos. 3,902,144, 4,933,377,4,760,013, 4,734,444, and 2833827; diazonium salts (e.g., benzenediazonium salts which may have one or more substituted groups);diazonium salt resins (e.g., formaldehyde resins of diazodiphenylamine);N-alkoxypyridinium salts (e.g., those described in U.S. Pat. No.4,743,528, JP-A Nos. 63-138345, 63-142345, and 63-142346, and JP-B No.4642363, and specifically, 1-methoxy-4-phenylpyridiniumtetrafluoroborate, etc.); and the compounds described in JP-B Nos.52-147277, 52-14278, and 52-14279 are suitably used. The aromatic oniumsalt compound (II) generates a radical or an acid as an active species.

Examples of the organic peroxides (iii) include almost all organiccompounds having one or more oxygen-oxygen bonds in the molecule; andpreferable examples thereof include peroxide esters such as3,3′,4,4′-tetra-(t-butylperoxycarbonyl)benzophenone,3,3′,4,4′-tetra-(t-amylperoxycarbonyl)benzophenone,3,3′,4,4′-tetra-(t-hexylperoxycarbonyl)benzophenone,3,3′,4,4′-tetra-(t-octylperoxycarbonyl)benzophenone,3,3′,4,4′-tetra-(cumylperoxycarbonyl)benzophenone,3,3′,4,4′-tetra-(p-isopropyl cumylperoxycarbonyl)benzophenone, anddi-t-butyl diperoxyisophthalate.

Examples of the hexaarylbiimidazole compounds (iv) include the Rofindimers described in JP-B Nos. 45-37377 and 44-86516, such as2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole,2,2′-bis(o-bromophenyl)-4,4′,5,5′-tetraphenylbiimidazole,2,2′-bis(o-,p-dichlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(m-methoxyphenyl)biimidazole,2,2′-bis(o-,o′-dichlorophenyl)-4,4′,5,5′-tetraphenylbimidazole,2,2′-bis(o-nitrophenyl)-4,4′,5,5′-tetraphenylbiimidazole,2,2′-bis(o-methylphenyl)-4,4′,5,5′-tetraphenylbiimidazole, and2,2′-bis(o-trifluorophenyl)-4,4′,5,5′-tetraphenylbiimidazole.

Examples of the ketoxime ester compounds (v) include3-benzoyloxyiminobutan-2-one, 3-acetoxyiminobutan-2-one,3-propionyloxyiminobutan-2-one, 2-acetoxyiminopentane-3-one,2-acetoxyimino-1-phenylpropan-1-one,2-benzoyloxyimino-1-phenylpropan-1-one,3-p-toluenesulfonyloxyiminobutan-2-one, and2-ethoxycarbonyloxyimino-1-phenylpropan-1-one.

Examples of the borate compounds (vi) include the compounds described inU.S. Pat. Nos. 3,567,453 and 4,343,891, and EP Patent No. 109,772 and109,773.

Examples of the azinium compounds (vii) include the compounds containingan N—O bond described in JP-A Nos. 63-138345, 63-142345, 63-142346, and63-143537, and JP-B No. 46-42363.

Examples of the metallocene compounds (viii) include the titanocenecompounds described in JP-A Nos. 59-152396, 61-151197, 63-41484, 2-249,and 2-4705 and the iron-allene complexes described in JP-A Nos. 1-304453and 1-152109.

Examples of the titanocene compounds includedi-cyclopentadienyl-Ti-dichloride, di-cyclopentadienyl-Ti-bisphenyl,di-cyclopentadienyl-Ti-bis-2,3,4,5,6-pentafluorophen-1-yl,di-cyclopentadienyl-Ti-bis-2,3,5,6-tetrafluorophen-1-yl,di-cyclopentadienyl-Ti-bis-2,4,6-trifluorophen-1-yl,di-cyclopentadienyl-Ti-2,6-difluorophen-1-yl,di-cyclopentadienyl-Ti-bis-2,4-difluorophen-1-yl,di-methylcyclopentadienyl-Ti-bis-2,3,4,5,6-pentafluorophen-1-yl,di-methylcyclopentadienyl-Ti-bis-2,3,5,6-tetrafluorophen-1-yl,di-methylcyclopentadienyl-Ti-bis-2,4-difluorophen-1-yl,bis(cyclopentadienyl)-bis(2,6-difluoro-3-(pyr-1-yl)phenyl)titanium,bis(cyclopentadienyl) bis[2,6-difluoro-3-(methylsulfonamide)phenyl]titanium, and bis(cyclopentadienyl)bis[2,6-difluoro-3-(N-butylbiaroyl-amino)phenyl)titanium.

Examples of the active ester compounds (ix) include nitrobenzyl estercompounds described in EP Patent Nos. 0290750, 046083, 156153, 271851,and 0388343, U.S. Pat. Nos. 3,901,710 and 4,181,531, and JP-A Nos.60-198538 and 53-133022; iminosulfonate compounds described in EP PatentNos. 0199672, 84515, 199672, 044115, and 0101122, U.S. Pat. Nos.4,618,564, 4,371,605, and 4,431,774, and JP-A Nos. 64-18143, 2-245756and 4-365048; and compounds described in JP-B Nos. 62-6223 and 63-14340,and JP-A No. 59-174831.

Examples of the compounds (x) containing a carbon-halogen bond includecompounds described in Wakabayashi et al., Bull. Chem. Soc. Japan, 42,2924 (1969), compounds described in British Patent No. 1388492,compounds described in JP-A No. 53-133428, and compounds described inGerman Patent No. 3337024.

Further examples of the compounds containing a carbon-halogen bondinclude compounds described in F. C. Schaefer et al., J. Org. Chem. 29,1527 (1964), compounds described in JP-A No. 62-58241, compoundsdescribed in JP-A No. 5-281728, compounds described in German Patent No.2641100, compounds described in German Patent No. 3333450, compoundsdescribed in German Patent No. 3021590, and compounds described inGerman Patent No. 3021599.

Preferable specific examples of the compounds represented by (i) to (x)described above are shown below.

Only one photopolymerization initiator may be used, or two or morephotopolymerization initiators may be used in combination. The contentof the photopolymerization initiator in the ink composition ispreferably 0.1% by mass to 20% by mass, more preferably 0.5% by mass to10% by mass, most preferably 1% by mass to 7% by mass, based on thetotal solid content in the ink composition.

<Other Component>

—Sensitizing Dye—

In the present invention, a sensitizing dye may be added for the purposeof improving the sensitivity of the photopolymerization initiator. Forthe sensitizing dye, those are preferable that belong to the followingcompound classes and have absorption wavelengths in the range of 350 nmto 450 nm.

Examples of the sensitizing dye include multinuclear aromatics (e.g.,pyrene, perylene, triphenylene, and anthracene), xanthenes (e.g.,fluorescein, eosin, erythrosine, rhodamine B, and Rose Bengal), cyanines(e.g., thiacarbocyanine and oxacarbocyanine), merocyanines (e.g.,merocyanine and carbomerocyanine), thiazines (e.g., thionine, methyleneblue, and toluidine blue), acridines (e.g., acridine orange,chloroflavin, and acryflavin), anthraquinones (e.g., anthraquinone),squaliums (e.g., squalium), and coumarins (e.g., 7-diethylamino-4-methylcoumarin).

For the sensitizing dye, compounds represented by the following formulae(IX) to (XIII) are more preferable.

In formula (IX), A¹ represents a sulfur atom or —NR⁵⁰—, R⁵⁰ representsan alkyl group or an aryl group, L² represents a nonmetallic atomicgroup which, together with A¹ and the carbon atom adjacent thereto,forms a basic nucleus of a dye, R⁵¹ and R⁵² each independently representa hydrogen atom or a monovalent nonmetallic atomic group, R⁵¹ and R⁵²may be bound to each other to form an acidic nucleus of a dye, and Wrepresents an oxygen atom or a sulfur atom.

In formula (X), Ar¹ and Ar² each independently represent an aryl group,and are bound to each other via a linkage -L³- which represents —O— or—S—. W has the same definition as in formula (IX).

In formula (XI), A² represents a sulfur atom or NR⁵⁹, L⁴ represents anonmetallic atomic group which, together with A² and the carbon atomadjacent thereto, forms a basic nucleus of a dye, R⁵³, R⁵⁴, R⁵⁵, R⁵⁶,R⁵⁷ and R⁵⁸ each independently represent a monovalent nonmetallic atomicgroup, and R⁵⁹ represents an alkyl group or an aryl group.

In formula (XII), A³ and A⁴ each independently represent —S— or —NR⁶²—or —NR⁶³—. R⁶² and R⁶³ each independently represent a substituted orunsubstituted alkyl group or a substituted or unsubstituted aryl group.L⁵ represents a nonmetallic atomic group which, together with A³ and thecarbon atom adjacent thereto, forms a basic nucleus of a dye. L⁶represents a nonmetallic atomic group which, together with A⁴ and thecarbon atom adjacent thereto, forms a basic nucleus of a dye. R⁶⁰ andR⁶¹ each independently represent a hydrogen atom or a monovalentnonmetallic atomic group, or R⁶⁰ and R⁶¹ are bound to each other to forman aliphatic or aromatic ring.

In formula (XIII), R⁶⁶ represents an optionally substituted aromaticcycle or heterocycle, A⁵ represents an oxygen atom, a sulfur atom, or—NR⁶⁷—. R⁶⁴, R⁶⁵, and R⁶⁷ each independently represent a hydrogen atomor a monovalent nonmetallic atomic group. R⁶⁷ and R⁶⁴ may be bonded toeach other to form an aliphatic or aromatic ring. R⁶⁵ and R⁶⁷ may bebonded to each other to form an aliphatic or aromatic ring.

Preferable specific examples of the compounds represented by formulae(IX) to (XIII) include exemplary compounds (A-1) to (A-24) shown below.

—Cosensitizer—

To the ink composition of the present invention, a known compound thathas a function of further improving the sensitivity or suppressing theinhibition of polymerization by oxygen may be added as a cosensitizer.

Examples of the cosensitizer include the amines described, for example,in M. R. Sander et al., “Journal of Polymer Society” 10, p. 3173,(1972), JP-B No. 44-20189, JP-A Nos. 51-82102, 52-134692, 59-138205,60-84305, 62-18537, and 64-33104 and Research Disclosure 33825; andspecific examples thereof include triethanolamine, ethylp-dimethylaminobenzoate, p-formyldimethylaniline, andp-methylthiodimethylaniline.

Other examples of the cosensitizer include thiols and sulfides, forexample, the thiol compounds described in JP-A No. 53-702, JP-B No.55-500806, and JP-A No. 5-142772, and the disulfide compounds describedin JP-A No. 56-75643; and specific examples thereof include2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole,2-mercapto-4(3H)-quinazoline, and b-mercaptonaphthalene.

Yet other examples of the cosensitizer include amino acid compounds(e.g., N-phenylglycine), the organic metal compounds described in JP-BNo. 4842965 (e.g., tributyltin acetate), the hydrogen donors describedin JP-B No. 55-34414, the sulfur compounds described in JP-A No.6-308727 (e.g., trithiane), the phosphorus compounds described in JP-ANo. 6-250387 (e.g., diethyl phosphite), and the Si—H and Ge—H compoundsdescribed in JP-A No. 6-191605.

In addition to the essential components, the other polymerizablecompound (a) and (b) pigment, a preferable optional component, thepolymerization initiator (c), and the sensitizing dye and cosensitizerthat can be used together with the polymerization initiator (c), the inkcomposition according to the present invention may further containvarious additives according to the purpose. For example, an ultravioletabsorbent may be added to the ink composition according to the presentinvention, for the improvement in the weather resistance of the obtainedimage and prevention of the discoloration of the image. In addition, anantioxidant may be added to improve the stability of the inkcomposition.

Other usable additives include: an organic or metal-complex-basedanti-fading agent; a conductive salt for the control of the ejectionproperties, such as potassium thiocyanate, lithium nitrate, ammoniumthiocyanate, or dimethylamine hydrochloride salt; and a trace amount ofan organic solvent for the improvement of the adhesion to the recordingmedium.

The ink composition according to the present invention may furthercontain a polymer compound selected from various polymer compounds forthe purpose of the adjustment of the film physical properties. Examplesof polymer compounds include acrylic polymers, polyvinylbutyral resins,polyurethane resins, polyamide resins, polyester resins, epoxy resins,phenol resins, polycarbonate resins, polyvinylbutyral resins,polyvinylformal resins, shellac, vinyl resins, acrylic resins, rubberresin, waxes, and other natural resins. Two or more polymer compoundsmay be used simultaneously.

One or more substances selected from nonionic surfactants, cationicsurfactants, and organic fluorocompounds may be added to the inkcomposition of the present invention in order to control the liquidproperties.

The ink composition of the present invention may contain other additivesas necessary, for example, leveling additives, matting agents, waxes forcontrolling the film properties, and tackifiers, which do not inhibitpolymerization, for improving the adhesion to recording media such aspolyolefin and PET.

The ink composition of the present invention has a viscosity ofpreferably 30 mPa·s or less and more preferably 20 mPa·s or less at thetemperature of injection taking injection ability into account. It ispreferable to control and determine the ratio of the compositionproperly such that the viscosity of the ink composition falls in theabove range. It is to be noted that the viscosity of the ink at 25° C.(room temperature) is 200 mPa·s or less and preferably 100 mPa·s. Bysetting the viscosity high at room temperature, even if a porousrecording medium is used, it is possible to prevent permeation of inkinto the recording medium, reduce the amount of an uncured monomer, anddecrease odors, and further, blurring of dots when ink droplets areimpacted can be suppressed, resulting in improved image quality. Whenthe viscosity of the ink at 25° C. is higher than 200 mPa·s, a problemarises concerning the delivery of the ink liquid.

The surface tension of the ink composition according to the presentinvention is preferably 20 mN/m to 30 mN/m and more preferably 23 mN/mto 28 mN/m. When the ink is used for recording on various recordingmedia such as polyolefin, PET, coated paper, and non-coated paper, thesurface tension is preferably 20 mN/m or more in view of the preventionof bleeding and penetration, and 30 mN/m or less in view of thewettability.

The ink composition of the present invention prepared in this manner issuitably used as an inkjet recording ink. The recording can be conductedby applying the ink composition on a recording medium by printing usingan inkjet printer, and then irradiating the applied ink composition witha radiation ray to cure the composition.

In the printed articles obtained by the ink, the image portion has beencured by irradiation with radiation rays such as ultraviolet rays andthe printed articles are therefore superior in the strength of the imageportion. Therefore, the ink composition of the present invention may beused in various applications such as the formation of the ink receptorlayer (image portion) of a planographic printing plate, besides imageformation using ink.

(Inkjet Recording Method and Inkjet Recording Apparatus)

Next, an inkjet recording method and an inkjet recording apparatus,which are suitably adopted in the present invention, will be describedbelow.

In the inkjet recording method, it is preferable to eject the inkcomposition after the viscosity of ink composition is lowered to 30mPa·s or less by heating to 40° C. to 80° C., and in this manner, it ispossible to realize highly stable ejection. Radiation-curable inkcompositions are usually more viscous than aqueous inks. Thus,generally, fluctuation in the viscosity of radiation-curable inkcompositions caused by the fluctuation in temperature during printing islarger. The fluctuation in the viscosity of ink composition exertssignificant influences on the droplet size and the droplet ejectionspeed, causing deterioration in image quality, and thus, it is necessaryto keep the temperature of the ink composition as constant as possibleduring printing. It is preferable to control the ink compositiontemperature within 5° C. from the set temperature, more preferably 2° C.from the set temperature, and most preferably 1° C. from the settemperature.

The inkjet recording apparatus may have an ink-temperature stabilizingdevice. The ink-temperature stabilizing device maintains a constanttemperature of the ink composition in all the piping systems and membersfrom the ink tank (from the intermediate tank if such an intermediatetank is present) to the ejection face on the nozzles.

The temperature can be controlled by any method without limitation. Forexample, it is preferable to control heating conditions according to theflow rate of the ink composition and the environmental temperature basedon the information supplied from plural temperature sensors provided tothe respective pipes. The heat unit to be heated is preferably insulatedthermally such that the unit is not affected by the environmentaltemperature. Preferably, the heat unit is thermally insulated from theother portions, and the total heat capacity of the heating unit issmall, whereby the printer starting-up time required for heating isshortened and the heat energy loss is reduced.

An active radiation-curable ink composition can be obtained by addingthe photopolymerization initiator as the polymerization initiator (c) tothe ink composition of the present invention.

Conditions of the irradiation of the ink composition with an activeradiation ray will be described below. A basic method of the irradiationis disclosed in JP-A No. 60-132767. Specifically, light sources aredisposed at both sides of a head unit, and the head unit and the lightsources are scanned in the shuttle mode. The ink composition isirradiated with the active radiation ray after a predetermined periodfrom the deposition of the ink composition on the recording medium. Theink composition is cured using another light source that is not driven.Specifically, WO 99/54415 discloses an irradiation method comprisingusing an optical fiber and an irradiation method comprising irradiatingthe recording area with UV rays by directing a collimated rays to amirror surface on the sidewall of head unit. These irradiation methodsmay be used in the present invention.

Further, in the present invention, it is desirable to heat the inkcomposition to a predetermined temperature and adjust the period betweenthe deposition of the ink composition on the recording medium and theirradiation with radiation rays to 0.01 second to 0.5 second, preferably0.01 second to 0.3 second, and more preferably 0.01 second to 0.15second. It becomes possible to prevent bleeding of the deposited inkcomposition before curing, by shortening the period between thedeposition of the ink composition on the recording medium and theirradiation with radiation rays to such an extremely short period.Further, even when the recording medium is porous, the ink compositionis irradiated before penetrating deep into the recording medium. Thus,the amount of the remaining unreacted monomer is reduced andconsequently the odor is also reduced. The combination of the inkjetrecording method and the ink composition of the present inventionprovides significant synergy effects. In particular when the viscosityof the ink composition at 25° C. is 200 mPa·s or less, significanteffects can be obtained. By employing such an inkjet recording method,it is possible to maintain the dot diameter of the deposited inkcomposition constant and obtain an image with improved quality, on anyof various recording media different in surface wettability. In order toobtain a color image, it is preferable to form images in the order froma color lower in lightness. When an ink of lower lightness is deposited,the radiation rays are unlikely to reach the inks located at the bottom;therefore, curing sensitivity and improvement in adhesiveness are likelyto be deteriorated, and the residual monomer is likely to be increasedto cause odor. Although it is possible to eject inks of all colors andthen conduct the irradiation at the same time, it is preferable toirradiate the image with radiation rays after each color ink isdeposited, in view of the acceleration of curing.

The inkjet recording apparatus used in the present invention is notparticularly limited, and a commercial inkjet recording apparatus can beused. In other words, in the present invention, the recording onrecording media (printed articles) can be conducted by a commercialinkjet recording apparatus.

In the preferable ejecting conditions described above, although the inkcomposition of the present invention is repeatedly heated and cooled,reduction in pigment dispersibility is avoided, excellent coloringproperty is achieved over a long period, and the deterioration of theejection property caused by the aggregation of the pigment is alsoavoided owing to the function of the specific polymer that contains acopolymer unit derived from the monomers represented by the formula (1)to (3) even when the ink composition is stored under such temperatureconditions.

(Recording Medium (Printed Articles))

The recording medium to which the ink composition according to thepresent invention is applicable is not particularly limited, andexamples thereof include ordinary papers such as non-coated paper,coated paper, and corrugated paper, various non-absorptive resinmaterials for use in so-called soft packaging, and resin films thereofin the film shape. Examples of such various plastic films include PETfilm, OPS film, OPP film, ONy film, PVC film, PE film, and TAC film.Examples of other plastics usable as the material of the recordingmedium include polycarbonate, acrylic resins, ABS, polyacetal, PVA, andrubbers. In addition, metals and glasses are also usable as therecording media. Recording media having a large area may also be used.

The ink composition of the present invention is used to print on arecording medium by an inkjet printer and then, preferably, the inkcomposition applied by printing is irradiated with an active radiationray and cured, whereby the printed articles of the present invention canbe obtained. The printed article of the present invention has a highquality image superior in coloring property and sharpness and is alsosuperior in the weather resistance of the image because the ink used forimage formation contains fine pigment particles uniformly and stably ina dispersed state. The ink composition of the invention is thereforeapplied in a wide range of fields.

EXAMPLES

The present invention will be explained in more detail by way ofExamples, which are not intended to limit the present invention.

Example 1 Synthesis of Polymerizable Compound and Polymer Thereof

—Synthesis of M-1—

355.0 g of 1,8-naphthalimide was dissolved in 1,500 mL ofN-methylpyrrolidone, and 0.57 g of nitrobenzene was added at 25° C.301.4 g of diazabicycloundecene (DBU) was added dropwise to the mixture.After stirring for 30 minutes, 412.1 g of chloromethylstyrene was addeddropwise to the mixture, which was then heated at 60° C. with stirringfor 4 hours. 2.7 L of isopropanol and 0.9 L of distilled water wereadded to this reaction solution and stirred while cooling to 5° C. Theresulting precipitates were separated by filtration and washed with 1.2L of isopropanol to obtain 544.0 g of polymerizable compound M-1.

From NMR data shown below, the compound obtained was confirmed to bepolymerizable compound M-1.

1H-NMR (300 MHz, CDCl₃, δ): 8.61 (d, 2H), 8.21 (d, 2H), 7.75 (t, 2H),7.25-7.60 (m, 4H), 6.66 (m, 1H), 5.73 (dd, 1H), 5.38 (s, 2H), 5.20 (dd,1H).

—Synthesis of M-3—

19.7 g of 1,8-naphthalimide was dissolved in 100 mL ofN-methylpyrrolidone, 11.1 g of triethylamine was added and stirred. 20.2g of 4-vinylbenzene sulfonyl chloride was added dropwise to the mixtureand stirred for 2 hours. 500 mL of distilled water was added to thisreaction solution and stirred while cooling to 5° C. The resultingprecipitates were separated by filtration and washed with methanol toobtain 20.0 g of polymerizable compound M-3.

From NMR data shown below, the compound obtained was confirmed to bepolymerizable compound M-3.

1H-NMR (300 MHz, CDCl₃, δ): 8.61 (d, 2H), 8.21 (d, 2H), 8.01 (d, 2H),7.75 (t, 2H), 7.60 (d, 2H), 6.76 (m, 1H), 5.93 (dd, 1H), 5.58 (dd, 1H),5.40 (d, 2H).

—Synthesis of Graft Copolymer 1—

2.0 g of M-1, 18.0 g of polymethyl methacrylate having a methacryloylgroup at one terminal (AA-6, manufactured by Toagosei Co., Ltd.), and 20g of methyl ethyl ketone were introduced into a three-neck flask inwhich the atmosphere was substituted with nitrogen, stirred with astirrer (Three-one Motor, manufactured by Shinto Scientific Co., Ltd.)and heated to raise the temperature to 78° C. with nitrogen flowing inthe flask. 27 mg of 2,2-azobis(2,4-dimethylvaleronitrile) (V-65,manufactured by Wako Pure Chemical Industries, Ltd.) was added to theabove solution and heated at 78° C. with stirring for 2 hours. After 2hours, 27 mg of V-65 was further added and heated with stirring for 3hours. The resulting reaction solution was poured into 1,000 mL ofhexane with stirring, and the precipitates produced were heated anddried to obtain a graft copolymer 1 (the above listed graft copolymer 1:A copolymer of the monomer represented by M-1, andpolymethylmethacrylate having a methacryloyl group at its terminal).

The weight average molecular mass (polystyrene standard) of the graftcopolymer 1, determined by GPC, was 31,500. Thus, it was confirmed thatpolymer was obtained.

For GPC, Shodex GPC KF-804 (Showa Denko K.K.) was used as a column, andTHF was used as an eluent. Measurement was performed at a flow rate of0.8 mL/min and a column temperature of 40° C., and refractive index (RI)detection was used.

—Synthesis of Graft Copolymer 2—

2.0 g of M-1, 2.0 g of 3-(N,N-dimethylaminopropyl acrylamide), 16.0 g ofpolymethyl methacrylate having a methacryloyl group at one terminal(AA-6, manufactured by Toagosei Co., Ltd.), and 20 g of methyl ethylketone were introduced into a three-neck flask in which the atmospherewas substituted with nitrogen, stirred with a stirrer (Three-one Motor,manufactured by Shinto Scientific Co., Ltd.) and heated to raise thetemperature to 78° C. with nitrogen flowing in the flask. 27 mg of2,2-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Wako PureChemical Industries, Ltd.) was added to the above solution and heated at78° C. with stirring for 2 hours. After 2 hours, 27 mg of V-65 wasfurther added and heated with stirring for 3 hours. The resultingreaction solution was poured into 1,000 mL of hexane with stirring, andthe precipitates produced were heated and dried to obtain a graftcopolymer 2 (the above listed graft copolymer 8: A copolymer of themonomer represented by M-1,3-(N,N-dimethylamino)propyl acrylamide, andpolymethylmethacrylate having a methacryloyl group at its terminal).

The weight average molecular mass (polystyrene standard) of the graftcopolymer 2, determined by GPC, was 56,600. Thus, it was confirmed thatpolymer was obtained.

—Synthesis of Graft Copolymer 3—

A graft copolymer 3 (the above listed graft copolymer 2: A copolymer ofthe monomer represented by M-1, and polyethylene glycolmono(meth)acrylate) was obtained in the same way as in the syntheticexample of graft copolymer 1 except that the “polymethyl methacrylatehaving a methacryloyl group at one terminal” of the synthetic example ofgraft copolymer 1 was changed to “methoxypolyethylene glycolmethacrylate” (NK ESTER M-230G, manufactured by Shin-Nakamura ChemicalCo., Ltd.).

The weight average molecular mass (polystyrene standard) of the graftcopolymer 3, determined by GPC, was 24,500. Thus, it was confirmed thatpolymer was obtained.

—Synthesis of Graft Copolymer 4—

A graft copolymer 4 (the above listed graft copolymer 4: A copolymer ofthe monomer represented by M-1, and polybutyl acrylate having amethacryloyl group at its terminal) was obtained in the same way as inthe synthetic example of graft copolymer 1 except that the “polymethylmethacrylate having a methacryloyl group at one terminal” of thesynthetic example of graft copolymer 1 was changed to “polybutylacrylate having a methacryloyl group at one terminal” (AB-6,manufactured by Toagosei Co., Ltd.).

The weight average molecular mass (polystyrene standard) of the graftcopolymer 4, determined by GPC, was 28,800. Thus, it was confirmed thatpolymer was obtained.

—Synthesis of Graft Copolymer 5—

A graft copolymer 5 (the above listed graft copolymer 5: A copolymer ofthe monomer represented by M-3, and polymethylmethacrylate having amethacryloyl group at its terminal) was obtained in the same way as inthe synthetic example of graft copolymer 1 except that M-1 of thesynthetic example of graft copolymer 1 was changed to M-3.

The weight average molecular mass (polystyrene standard) of the graftcopolymer 5, determined by GPC, was 18,700. Thus, it was confirmed thatpolymer was obtained.

—Synthesis of Graft Copolymer 6

A graft copolymer 6 (the above listed graft copolymer 16: A copolymer ofthe monomer represented by M-1, methacrylic acid, andpolymethylmethacrylate having a methacryloyl group at its terminal) wasobtained in the same way as in the synthetic example of graft copolymer2 except that the “3-(N,N-dimethylaminopropyl acrylamide)” of thesynthetic example of graft copolymer 2 was changed to “methacrylicacid”.

The weight average molecular mass (polystyrene standard) of the graftcopolymer 6, determined by GPC, was 19,800. Thus, it was confirmed thatpolymer was obtained.

Graft copolymer 1 (the graft copolymer 1 listed above: copolymer of themonomer, represented by M-1, and polymethylmethacrylate having amethacryloyl group at its terminal) was dissolved in other polymerizablecompounds (a), (i) and (ii), and the mixture was placed together with apigment (b) in a Motor Mill M50 (manufactured by Eiger Co., Ltd.) todisperse the mixture at a peripheral speed of 9 m/s for 6 hours by usingzirconia beads 0.65 mm in diameter, thereby obtaining a reaction mixtureof active energy ray-curable ink. Then, the polymerization initiator (c)was added to the reaction mixture and mixed mildly, followed byfiltration under pressure using a membrane filter to obtain an activeenergy ray-curable type inkjet ink of Example 1. The amounts of graftcopolymer 1, other polymerizable compounds (a), (i) and (ii), pigment(b), and polymerization initiator (c) added are shown below.

Graft copolymer 1 1.5 g (a) Other polymerizable compound: (i)propoxylated neopentyl glycol diacrylate (NPGPODA, 60.0 g  manufacturedby Sartomer Company, Inc.) (ii) caprolactone-modified dipentaerythritolhexaacrylate 27.5 g  (DPCA-60, manufactured by Nippon Kayaku Co., Ltd.)(b) Pigment: quinacridone base pigment PV-19 5.0 g (c) Polymerizationinitiator: acylphosphine oxide compound 5.0 g (LucirinTPO-L,manufactured by BASF)

Examples 2 to 6

Active energy ray-curable type inkjet inks of Examples 2 to 6 wereobtained in the same way as in Example 1 except that the graft copolymer1 used in Example 1 was changed to the graft copolymers 2 to 6,respectively, prepared in the synthetic examples.

Comparative Example 1

An active energy ray-curable type inkjet ink of Comparative Example 1was obtained in the same way as in Example 1 except that a commerciallyavailable pigment dispersant “SOLSPERSE 24000GR” (manufactured byLubrizol Japan Ltd.) was used instead of the graft copolymer 1 used inExample 1.

Comparative Example 2

An active energy ray-curable type inkjet ink of Comparative Example 2was obtained in the same way as in Example 1 except that a commerciallyavailable pigment dispersant “SOLSPERSE 32000” (manufactured by LubrizolJapan Ltd.) was used instead of the graft copolymer 1 used in Example 1.

Example 7 and Comparative Examples 3 and 4

Inkjet ink compositions of Example 7 and Comparative Examples 3 and 4were obtained in the same way as in Example 2 and Comparative Examples 1and 2, respectively, except that the quinacridone base pigment PV-19 (b)was changed to PY-128.

Example 8 and Comparative Examples 5 and 6

Inkjet inks of Example 8 and Comparative Examples 5 and 6 were obtainedin the same way as in Example 2 and Comparative Examples 1 and 2,respectively, except that the other polymerizable compound (a) andpolymerization initiator (c) were changed to the following compounds.

(a) Other polymerizable compound: (i) oxetane compound (OXT-221,manufactured by Toagosei 70.0 g Co., Ltd.) (ii) epoxy compound (Celoxide3000, manufactured by Daicel 17.5 g Chemical Industries, Ltd.) (c)Polymerization initiator: triphenylsulfonium salt (UVI-6992,  5.0 gmanufactured by Dow Chemical)

Example 9 and Comparative Examples 7 and 8

Inkjet inks of Example 9 and Comparative Examples 7 and 8 were obtainedin the same way as in Example 8 and Comparative Examples 5 and 6,respectively, except that the quinacridone base pigment PV-19 waschanged to a phthalocyanine-based pigment PB15:3.

Example 10

The polymer of the present invention shown below (graft copolymer 2) wasdissolved in the following polymerizable compounds (a), and the mixturewas placed together with the following pigment (b) in a Motor Mill M50(manufactured by Eiger Co., Ltd.) to disperse the mixture at aperipheral speed of 5 m/s for 5 hours by using zirconia beads 0.65 mm indiameter, thereby obtaining a reaction mixture of active energyray-curable ink. Then, a polymerization initiator (c) was added to thereaction mixture and mixed mildly, followed by filtration under pressureusing a membrane filter to obtain an inkjet ink of Example 10.

(b) Pigment [acetolone pigment PO-36 KENALAKE ORANGE 4.0 g HPRLO,manufactured by Albion Colours] Polymer of the present invention [graftcopolymer 2 obtained 1.2 g in the synthetic example] (a) Polymerizablecompound [propoxylated neopentyl glycol 40.0 g  diacrylate (NPGPODA,manufactured by Sartomer Company, Inc. (a) Polymerizable compound[dipropylene glycol diacrylate 49.8 g  (DPGDA, manufactured byDAICEL-CYTEC Company Ltd.] (c) Polymerization initiator [acylphosphineoxide 5.0 g compound (LucirinTPO-L: manufactured by BASF)]

Comparative Example 9

An inkjet ink of Comparative Example 9 was obtained in the same way asin Example 10 except that the polymer of the present invention (graftcopolymer 2) used in Example 10 was changed to a commercially availablepigment dispersant “SOLSPERSE 24000GR” (manufactured by Lubrizol JapanLtd.).

Example 11 and Comparative Example 10

Inkjet inks of Example 11 and Comparative Example 10 were obtained inthe same way as in Example 10 and Comparative Example 9, respectively,except that the pigment (b) used in Example 10 and Comparative Example 9was changed to acetolone pigment (PY-120, Novoperm Yellow H2G,manufactured by Clariant(Japan)K.K.).

Example 12

An inkjet ink of Example 12 was obtained in the same way as in Example10 except that the polymerizable compound (a), pigment (b),polymerization initiator (c) used in Example 10 were changed tofollowing compounds.

(b) Pigment [Dioxazine pigment PV-23, Hostaperm Violet BL] 3.0 g Polymerof the present invention [graft copolymer 2 obtained 0.9 g in thesynthetic example]) (a) Polymerizable compound [propoxylated neopentylglycol 40.0 g  diacrylate] (NPGPODA, manufactured by Sartomer Company,Inc.) (a) Polymerizable compound [dipropylene glycol diacrylate 50.6 g (DPGDA, manufactured by DAICEL-CYTEC Company Ltd. (c) Polymerizationinitiator [acylphosphine oxide 5.5 g compound (LucirinTPO-L:manufactured by BASF]

Comparative Example 11

An inkjet ink of Comparative Example 11 was obtained in the same way asin Example 12 except that the polymer of the present invention (graftcopolymer 2) used in Example 12 was changed to a commercially availablepigment dispersant “SOLSPERSE 24000GR” (manufactured by Lubrizol JapanLtd.).

(Evaluation of Ink Compositions)

The obtained inkjet inks were evaluated according to the followingmethods. The results are shown in Table 1.

—Viscosity—

The viscosity of each inkjet ink at 40° C. was measured with an E-typeviscometer.

A: 30 mPa·s or less.

B: exceeding 30 mPa·s and less than 100 mPa·s.

C: 100 mPa·s or more (practically problematic level with regard tojetting ability).

—Stability—

The state of dispersion of each inkjet ink after being stored at 25° C.for one month and after being stored at 60° C. for 24 hours wasevaluated visually and according to viscosity change.

A: No precipitate is generated, and an increase in viscosity is notobserved.

B: No precipitate is generated, and although a slight increase inviscosity is observed, it is a non-problematic level with regard tojetting ability.

C: No precipitate is generated, but an increase in viscosity isobserved, which is a practically problematic.

D: Precipitates are generated.

—Average Particle Diameter—

The volumetric average particle diameter D50 of each inkjet ink wasmeasured using a light-scattering diffraction type particle sizedistribution measuring device (LA910, manufactured by Horiba, Ltd.) tocarry out evaluation.

A: D50 is less than 100 nm.

B: D50 is 100 nm or more and less than 200 nm.

C: D50 is 200 nm or more.

—Curability—

The obtained ink compositions were applied to art paper by printingusing an inkjet printer (printing density: 300 dpi, dotting frequency: 4kHz, number of nozzles: 64) and then exposed to light by using a Deep UVlamp (SP-7, manufactured by Ushio Inc.) under an energy condition of 100mJ/cm² to obtain a print sample.

A cured coating film was touched with fingers to evaluate the presenceof a sticky feel according to the following standard.

TABLE 1 Evaluation of ink composition Stability (Room Stability ParticlePigment dispersant Viscosity temperature) (60° C.) diameter CurabilityExample 1 Polymer 1 A A A A A Example 2 Polymer 2 A A A A A Example 3Polymer 3 A A A A A Example 4 Polymer 4 A A A A A Example 5 Polymer 5 AA A A A Example 6 Polymer 6 A A B A A Comparative Commercially availableA C C A A Example 1 dispersant 24000GR Comparative Commerciallyavailable A A C A A Example 2 dispersant 32000 Example 7 Polymer 2 A A AA A Comparative Commercially available C C D C A Example 3 dispersant24000GR Comparative Commercially available B C C B A Example 4dispersant 32000 Example 8 Polymer 2 A A A A A Comparative Commerciallyavailable A C C B A Example 5 dispersant 24000GR ComparativeCommercially available A B C B A Example 6 dispersant 32000 Example 9Polymer 2 A A A A A Comparative Commercially available C C D C A Example7 dispersant 24000GR Comparative Commercially available B B D B AExample 8 dispersant 32000 Example 10 Polymer 2 A A A A A ComparativeCommercially available B C D B A Example 9 dispersant 24000GR Example 11Polymer 2 A B B A A Comparative Commercially available C C D C A Example10 dispersant 24000GR Example 12 Polymer 2 A A A A A ComparativeCommercially available C C D A A Example 11 dispersant 24000GR A: Nosticky feel. B: Slightly sticky. C: Very Sticky.

As is clear from Table 1, the ink composition of the present inventionwas cured with high sensitivity when irradiated with a radiation ray,and it could therefore form a non-sticky high quality image. Even in thecase where it was stored for a long time, the dispersibility anddispersion stability of the pigment were both satisfactory without anyincrease in viscosity associated with a reduction in the dispersibilityof the pigment.

On the other hand, although the comparative examples using commerciallyavailable high molecular dispersant had favorable pigment dispersibilityinitially, they were deteriorated, particularly, in storage under ahigh-temperature condition (stability (60° C.)), showing that they hadcharacteristics at a practically problematic level.

In the Examples described above, only data of Examples, in whichpolymers 1 to 6 were used as a pigment dispersant, are shown. However,it can be presumed that when the polymers of the present invention otherthan the polymers 1 to 6 are used as a pigment dispersant, they alsohave dispersion performance as the polymers 1 to 6 do. The reason forthis is that the polymer of the present invention comprises, as a basicbuilding block, a site interacting with a pigment, and a graft chain(macromonomer). Here, the site interacting with a pigment indicates thepolymerizable compound of the present invention, and, according tonecessity, also includes a basic monomer or acidic monomer (those whichinteract with acidic or basic site of the pigment surface). The graftchain (macromonomer) has a higher affinity for a dispersion medium thanfor a pigment and functions as a steric repulsive site that prevents theparticles from aggregating in the dispersion liquid. Thus, it isnecessary to select a graft chain depending on the dispersion medium.

The ink composition of the present invention can be suitably used inusual printing to form a sharp image with excellent coloring property,thus giving a high-quality printed material. The ink composition of thepresent invention can be suitably used also in the production ofresists, color filters, and optical disks, and is useful also as anoptical molding material.

In addition, since the application of the inkjet recording method makesit possible to form a high-quality image directly even on anon-absorptive recording medium based on digital data, the inkcomposition of the present invention can be suitably used also for theproduction of a printed material having a large area.

1. An ink composition comprising: a polymer; a polymerizable compound(a); and a pigment (b), wherein the polymer comprises a copolymer unitderived from a monomer represented by formula (1):

wherein, in the formula (1), R¹ represents any one of a hydrogen atom, asubstituted alkyl group, and an unsubstituted alkyl group; R² representsan alkylene group; W represents any one of —CO—, —C(═O)O—, —CONH—,—OC(═O)—, and an unsubstituted phenylene group; X represents any one of—CO—, —NHCO—, —OC(═O)—, —CH(OH)CH₂—, and —SO₂—; R³ and R⁴ eachindependently represents a monovalent substituent and the monovalentsubstituent is any one of a hydrogen atom, an alkyl group, an arylgroup, a halogen atom, an alkoxy group, an aryloxy group, an alkylthiogroup, an arylthio group, an alkoxycarbonyl group, an acylamino group, acarbamoyl group, a cyano group, an alkylsulfonyl group, an arylsulfonylgroup, an acyl group, a sulfamoyl group, an acyloxy group, a carboxylgroup, and a sulfonyl group; R³ and R⁴ do not bind to each other to forma ring structure; and m and n each independently represent 0 or
 1. 2.The ink composition according to claim 1, further comprising apolymerization initiator (c).
 3. The ink composition according to claim2, wherein the polymerizable compound (a) is a radical polymerizablecompound, and the polymerization initiator (c) is a photo-radicalgenerator.
 4. The ink composition according to claim 2, wherein thepolymerizable compound (a) is a cationic polymerizable compound, and thepolymerization initiator (c) is a photo-acid generator.
 5. The inkcomposition according to claim 1, wherein the polymer further comprisesa copolymer unit derived from a monomer represented by formula (2):

wherein, in the formula (2), R³ and R⁴ each independently represents amonovalent substituent and the monovalent substituent is any one of analkyl group, an aryl group, a halogen atom, an alkoxy group, an aryloxygroup, an alkylthio group, an arylthio group, an alkoxycarbonyl group,an acylamino group, a carbamoyl group, a cyano group, an alkylsulfonylgroup, an arylsulfonyl group, an acyl group, a sulfamoyl group, anacyloxy group, a carboxyl group, and a sulfonyl group; and R³ and R⁴ donot bind to each other to form a ring structure.
 6. The ink compositionaccording to claim 1, wherein the polymer is a graft copolymercontaining a copolymer unit derived from a macromonomer, wherein themacromonomer is a polymerizable oligomer having an ethylenicallyunsaturated double bond at a terminal thereof.
 7. The ink compositionaccording to claim 1, wherein the monomer represented by formula (1) isselected from the group consisting of the following compounds (M-1) to(M-12):


8. An inkjet recording method comprising: ejecting an ink composition ona recording medium using an inkjet printer; and irradiating the inkcomposition ejected with an active radiation ray to cure the inkcomposition, wherein the ink composition comprises a polymer; apolymerizable compound (a); and a pigment (b), wherein the polymercomprises a copolymer unit derived from a monomer represented by formula(1) as recited in claim 1.